I noticed that the apt repository includes mongodb-10gen-unstable, but only mongodb-10gen and mongodb18-10gen are documented on: http://docs.mongodb.org/manual/tutorial/install-mongodb-on-debian-or-ubuntu-linux/

As an aside, the mongodb-10gen-unstable seems to be out-of-date (it's a few releases behind the 2.2 release candidates). Perhaps this is no longer supported, or it's just fallen off the radar?