Ayende has blogged about a commercial product he is creating : the NHibernate profiler.
I’m really looking forward to seeing this tool in action – one of the difficult parts of using an ORM is tracking ‘how’ it is hitting the database, how sessions are created, for how long, etc…
Obviously, you can use SQL Profiler, but many times, your weeding through the calls, and getting all the visibility into the lifecycle’s of the sessions, etc…
But, go read – he has good pictures up that show how it works. 
I feel this tool will be a ‘must have’ for those using NHibernate on their projects.
