I just posted an article to TheCodeProject describing how to use Ayende's Rhino.Commons to connect together all the pieces of SqlCE and NHibernate to achieve disposable Database Unit Tests.
Amazingly the perfomance of SqlCE and NHibernate is very good. Once the initial SessionFactory is built, tests run pretty quickly. No benchmarks right now but I'll post them at a later date.
You can find the
article here
Enjoy!