NHibernate is a port of Hibernate Core for Java to the .NET Framework. It handles persisting plain .NET objects to and from an underlying relational database. Given an XML description of your entities and relationships, NHibernate automatically generates SQL for loading and storing the objects. Optionally, you can describe your mapping metadata with attributes in your source code.
Update 18-11-2008, well it turs out there infect is a NHibernate 2.0documentation here:
A very good place to start is here:
Chapter 5 explains setting up properties in the mapping file, tags and attributes.
Chapter 6 is about collections and lists
Handling many-to-many relations can be difficult, especially when it comes to cascading and data integrity. This blog post explains one way of handling it.