See http://is.gd/7OJiP for the relevant change.
"gem install rails3b" pulls in active_support-2.3.5 for me. These instructions should avoid that.
rails3_datamapper now depends on dm-active_model anyway.
As of this commit, my fork of datamapper is needed to deliver the promise that resources :person can now be replaced with resources :people However, I'm very positive that my changes find their way into upstream dm-core rather soon.