require 'merged_default_scopes' 

module SomePlugin
  def self.included(mod)
    mod.default_scope :conditions => {:deleted_at => nil}
  end
end

class SomeModel < ActiveRecord::Base
  include SomePlugin

  default_scope :order => :name
end

SomeModel.__send__(:scope, :find)
# => {:order => :name, :conditions => {:deleted_at => nil}}
#
# ... and all is right with the world

require 'merged_default_scopes' 

class SomeModel < ActiveRecord::Base
  include SomePluginThatMightSetDefaultScope
  include AnotherPluginThatMightSetDefaultScope

  clear_default_scope
  default_scope :order => :name
end

SomeModel.__send__(:scope, :find)
# => {:order => :name}
#
# ... and all is right with the world, assuming you want the old
# behaviour of default_scope, which is to overwrite previously set
# default scopes

$ gem install merged_default_scopes