What version is Fluent NHibernate designed for? The only officially supported version of NHibernate will always be the latest GA release. Current thinking is that we won't provide backwards compatibility outside of maintaining a tag in Subversion for the last revision before we upgrade.
What about NHibernate trunk? We realise a lot of people run on trunk, and we do provide a way for you to build against it (see our Rake Script), but we can't guarantee compatibility. If there are any breaking API changes in NHibernate trunk, then we won't be able to accommodate them; we are currently hashing over ideas for how to support those kind of situations without breaking our GA support.