If you are looking for a good introduction about NHibernate, Bob Palmer started a series of NHibernate tutorials as the first steps in a larger effort to help update the NHibernate reference documentation.
For many users, starting out with NHibernate is less of a learning curve and more of a learning cliff - so the intent with this first article series is to gradually expose the user to the features of NHibernate, from basic mapping through advanced techniques.
You can find the first three tutorials at these links:
Hello NHibernate! - Quickstart with NHibernate (Part 1)
Implementing a Repository with NHibernate - QuickStart with NHibernate (Part 2)
Mapping Object Relationships - QuickStart with NHibernate (Part 3)