NHibernate attributes can be used without issue instead of the regular hbm.xml mapping files. The one thing I don't like about them is that the attributes clutter up the class files somewhat. The other minor issue is that the documentation isn't nearly is good as Hibernate's EJB 3 implementation. Here's some documentation though:
http://nhibernate.sourceforge.net/nh-do ... butes.html
I believe there is a development version of Castle ActiveRecord that works with NHibernate 1.2 that you can probably pull from their source code repository. Ayende mentions that he has branched the ActiveRecord code base for NH1.2:
http://www.ayende.com/Blog/ForTheAdvent ... ate12.aspx
The latest Castle ActiveRecord release still only works against NH1.0.2.