I just uploaded an AndroMDA cartridge that generates NHibernate code from UML models. AndroMDA is an open source code generator that leverages Model Driven Architecture (MDA) to generated entire layers of an application from a single business model. If you have not used AndroMDA before, the learning curve to start using this cartridge may be a bit steep because it is not released as part of AndroMDA. However you can look at a sample application that is included to see the content and quality of the generated code. Check out this link for more details: http://galaxy.andromda.org/jira/browse/HIB-148. I would love to hear your feedback and whether something like this would be useful to you.

P.S. BTW I am using James' Nullable2 library to persist .NET 2.0 nullable types. Cool stuff. Thanks James.