Found this extremely useful blog on how to build an nhibernate custom provider for ASP.Net Membership role security.

http://lvildosola.blogspot.com/2007/02/nhibernateprovider-moves-to-codeplex.html

http://www.codeplex.com/nhibernateprovider/

Another individual trying to use nhibernate with membership providers

http://www.codeproject.com/KB/aspnet/eucalypto.aspx