The acts_as_taggable plugin is great and so useful. But it discards scope, and it doesn't work like any other model attribute in your controller views [using normal @model.update(params[:model]) calls].

I have changed it to:
* add an alias tag_list= for tag_with (found in the "rails wiki":http://wiki.rubyonrails.com/rails/pages/ActsAsTaggablePluginHowto),
* updated find_tagged_with and count_tagged_with to use scope (after reading an article from Jamis on "ActiveRecord::Base.find":http://weblog.jamisbuck.org/2006/11/20/under-the-hood-activerecord-base-find-part-2)

Scope is used when doing this for instance:

  Message.with_scope :find => {:conditions => ["project_id = ?", @project]} do
    @messages = Message.find_tagged_with(tag.name)
  end

Then only the messages tagged with tag.name that belong to project @project.


*acts_as_taggable.rb*

module ActiveRecord
  module Acts #:nodoc:
    module Taggable #:nodoc:
      def self.included(base)
        base.extend(ClassMethods)  
      end
      
      module ClassMethods
        def acts_as_taggable(options = {})
          write_inheritable_attribute(:acts_as_taggable_options, {
            :taggable_type => ActiveRecord::Base.send(:class_name_of_active_record_descendant, self).to_s,
            :from => options[:from]
          })
          
          class_inheritable_reader :acts_as_taggable_options

          has_many :taggings, :as => :taggable, :dependent => true
          has_many :tags, :through => :taggings

          include ActiveRecord::Acts::Taggable::InstanceMethods
          extend ActiveRecord::Acts::Taggable::SingletonMethods          
        end
      end
      
      module SingletonMethods
        def find_tagged_with(list)
          tagged_with list
        end

        def count_tagged_with(list)
          tagged_with list, :count
        end

        # DRY sql query and handle scope
        protected
        def tagged_with(list, type = :find)
          if type == :count
            sql = "SELECT COUNT(DISTINCT #{table_name}.#{primary_key}) AS cnt "
          else
            sql = "SELECT DISTINCT #{table_name}.* "
          end
          sql << "FROM #{table_name}, tags, taggings "

          conditions = [
            "#{table_name}.#{primary_key} = taggings.taggable_id " +
            "AND taggings.taggable_type = ? " +
            "AND taggings.tag_id = tags.id AND tags.name IN (?) ",
            acts_as_taggable_options[:taggable_type], list
            ]
          add_conditions!(sql, conditions)

          result = find_by_sql(sql)
          (type == :count) ? result.first.cnt.to_i : result
        end

      end
      
      module InstanceMethods
        def tag_with(list)
          Tag.transaction do
            taggings.destroy_all

            Tag.parse(list).each do |name|
              if acts_as_taggable_options[:from]
                send(acts_as_taggable_options[:from]).tags.find_or_create_by_name(name).on(self)
              else
                Tag.find_or_create_by_name(name).on(self)
              end
            end
          end
        end

        # allow using active record update_attributes() and others by using tag_list to set
        # and read the tag list
        alias tag_list= tag_with

        def tag_list
          tags.collect { |tag| tag.name.include?(" ") ? "'#{tag.name}'" : tag.name }.join(" ")
        end
      end
    end
  end
end

acts_as_taggable is cool, but it doesn't easily support limits, offsets, tag intersections, arbitrary conditions, etc.. so I created some unit tests and started to extend it. This is where I'm at so far:

module ActiveRecord
  module Acts #:nodoc:
    module Taggable #:nodoc:
      def self.included(base)
        base.extend(ClassMethods)  
      end
      
      module ClassMethods
        def acts_as_taggable(options = {})
          write_inheritable_attribute(:acts_as_taggable_options, {
            :taggable_type => ActiveRecord::Base.send(:class_name_of_active_record_descendant, self).to_s,
            :from => options[:from]
          })
          
          class_inheritable_reader :acts_as_taggable_options

          has_many :taggings, :as => :taggable, :dependent => true
          has_many :tags, :through => :taggings

          include ActiveRecord::Acts::Taggable::InstanceMethods
          extend ActiveRecord::Acts::Taggable::SingletonMethods          
        end
      end
      
      module SingletonMethods
        def find_tagged_with(list, options = {})
          local_options = { :limit => 1000, :offset => 0 }.merge(options)
          find_by_sql([
            "SELECT DISTINCT #{table_name}.* FROM #{table_name}, tags, taggings " +
            "WHERE #{table_name}.#{primary_key} = taggings.taggable_id " +
            "AND taggings.taggable_type = ? " +
            "AND taggings.tag_id = tags.id AND tags.name IN (?) #{"AND (#{local_options[:conditions]})" if local_options[:conditions]} LIMIT ? OFFSET ?",
            acts_as_taggable_options[:taggable_type], list, local_options[:limit], local_options[:offset]
          ])
        end
        
        def count_tagged_with(list, options = {})
          local_options = {}.merge(options)
          find_by_sql([
            "SELECT COUNT(DISTINCT #{table_name}.#{primary_key}) AS cnt FROM #{table_name}, tags, taggings " +
            "WHERE #{table_name}.#{primary_key} = taggings.taggable_id " +
            "AND taggings.taggable_type = ? " +
            "AND taggings.tag_id = tags.id AND tags.name IN (?) #{"AND (#{local_options[:conditions]}) " if local_options[:conditions]}",
            acts_as_taggable_options[:taggable_type], list
          ]).first.cnt.to_i
        end

        def find_tagged_with_intersecting(list, options = {})
          local_options = { :limit => 1000, :offset => 0 }.merge(options)
          find_by_sql([
            "SELECT DISTINCT #{table_name}.* FROM #{table_name}, tags, taggings " +
            "WHERE #{table_name}.#{primary_key} = taggings.taggable_id " +
            "AND taggings.taggable_type = ? " +
            "AND taggings.tag_id = tags.id AND tags.name IN (?) #{"AND (#{local_options[:conditions]}) " if local_options[:conditions]} GROUP BY #{table_name}.id HAVING COUNT(#{table_name}.id) = #{list.size} LIMIT ? OFFSET ?",
            acts_as_taggable_options[:taggable_type], list, local_options[:limit], local_options[:offset]
          ])
        end

        def count_tagged_with_intersecting(list, options = {})
          local_options = {}.merge(options)
          find_by_sql([
            "SELECT COUNT(*) AS cnt FROM (SELECT #{table_name}.#{primary_key} AS cnt FROM #{table_name}, tags, taggings " +
            "WHERE #{table_name}.#{primary_key} = taggings.taggable_id " +
            "AND taggings.taggable_type = ? " +
            "AND taggings.tag_id = tags.id AND tags.name IN (?) " +
            "#{"AND (#{local_options[:conditions]})" if local_options[:conditions]} " +
            "GROUP BY taggings.taggable_id HAVING COUNT(taggings.taggable_id) = #{list.size}) AS x",
            acts_as_taggable_options[:taggable_type], list
          ]).first.cnt.to_i
        end               
      end
      
      module InstanceMethods
        def tag_with(list)
          Tag.transaction do
            taggings.destroy_all

            Tag.parse(list).each do |name|
              if acts_as_taggable_options[:from]
                send(acts_as_taggable_options[:from]).tags.find_or_create_by_name(name).on(self)
              else
                Tag.find_or_create_by_name(name).on(self)
              end
            end
          end
        end

        def tag_list
          tags.collect { |tag| tag.name.include?(" ") ? "'#{tag.name}'" : tag.name }.join(" ")
        end
      end
    end
  end
end

These are the sorts of tests I'm then doing:

assert_equal 2, Post.find_tagged_with_intersecting(%w{TagOne TagTwo}).size
assert_equal 2, Post.count_tagged_with_intersecting(%w{TagOne TagTwo})
assert_equal 1, Post.find_tagged_with_intersecting(%w{TagOne TagTwo}, :conditions => 'status = 1').size
assert_equal 1, Post.count_tagged_with_intersecting(%w{TagOne TagTwo}, :conditions => 'status = 1')
p = Post.find_tagged_with("TagOne", :conditions => 'status = 1')