Specs surely don't pass, but they run at least!

lib/devise/orm/data_mapper.rb

@@ -72,7 +72,7 @@ module Devise
             end
           end
         end
-        
+
         def changed?
           dirty?
         end
@@ -84,7 +84,7 @@ module Devise
             super()
           end
         end
-        
+
         def update_attributes(*args)
           update(*args)
         end
@@ -93,7 +93,5 @@ module Devise
   end
 end
 
-DataMapper::Model.class_eval do
-  include Devise::Models
-  include Devise::Orm::DataMapper::Hook
-end
+DataMapper::Model.append_extensions(Devise::Models)
+DataMapper::Model.append_extensions(Devise::Orm::DataMapper::Hook)

test/orm/data_mapper.rb

@@ -1 +1,10 @@
+require 'rails/test_help'
+
 DataMapper.auto_migrate!
+
+class ActiveSupport::TestCase
+  setup do
+    User.all.destroy!
+    Admin.all.destroy!
+  end
+end

test/rails_app/app/data_mapper/shim.rb

@@ -0,0 +1,2 @@
+module Shim
+end

test/rails_app/app/data_mapper/user.rb

@@ -1,6 +1,9 @@
 class User
   include DataMapper::Resource
-  
+
+  extend  Devise::Orm::DataMapper::Schema
+  include Devise::Orm::DataMapper::Compatibility
+
   property :id, Serial
   property :username, String