I just filed a bug with one-to-one mapping using foreign keys with NHibernate that at least affects version 0.9.1 through 1.0.0. 

If you like one-to-one mappings, please visit the bug, download the
test case (which is a VS 2005 test project) and run it.  Evaluate
it for yourself and if you agree that it is a bug, please vote on it in
NHibernate’s JIRA.

Updated 11/21/05: This bug has been fixed and closed.  Thank you, NHibernate team!