My preferred approach to unit test DB code in any language is to access Mongo through a Repository abstraction (there's an example here http://iainjmitchell.com/blog/?p=884). Implementations will vary in terms of DB specific functionality exposed but by removing all the Mongo code from your own logic you're in a position to Unit Test. Simply replace the Mongo Repository implementation with a stubbed out version which is trivially easy. For instance, just store objects in a simple in-memory dictionary collection.
You'll get the benefits of unit testing your own code this way without DB dependencies but you'll still need to do integration tests against the main DB because you'll probably never be able to emulate the idiosyncrasies of the real database as others have said here. The kind of things I've found are as simple as indexing in safe mode vs without safe mode. Specifically, if you have a unique index your dummy memory implementation might honour that in all cases, but Mongo won't without safe-mode.
So whilst you'll still need to test against the DB for some operations, you'll certainly be able to unit test your own logic properly with a stubbed out Repository implementation.