Drupal 7 and MongoDB module howto
I (and my team) wrote most of that module and -- as Jeremy says, it's only needed for extreme scalability. Because of that, it's supposed you know what you are doing (otherwise you will have no chance of scaling to the levels where MongoDB is necessary) and so no UI is provided, just use $conf overrides.

There's a DBTNG driver too but it's more in a proof-of-concept stage so it's in a separate project.