Moving to NHibernate 2.0
Dario Quintana has something to say as well. He has contributed some code to the uNHAddins for Active Record Detached Query support.
The Castle development team is considering pointing to NHibernate Trunk (which will be released as 2.0 eventually) so they/we can take advantage of the new features.
If Castle Active Record uses NHibernate trunk, I would advise you to be diligent wit! your test code, and don't forget to run the Castle test code too.
You can track Castle Project changes on FishEye.