It's inaccurate that there's no difference between the global lock and database level locking. Whilst it's true that locking is now down at the database level and you get benefits from splitting into multiple databases, the real benefit is from the new PageFaultException architecture. Even using a single database you will see significant performance improvements.
I benchmarked this at http://blog.serverdensity.com/goodbye-global-lock-mongodb-2-... and there is a video explaining how this works at http://www.10gen.com/presentations/concurrency-internals-mon...