MongoDB is licensed under AGPL. There are some restrictions around AGPL, which, if you aren't happy with, you can buy a commercial license from 10gen.
Those restrictions are almost all about making changes to the core mongodb server. They aren't about _using_ mongodb.
So, unless you are making to the mongodb source code (recompiling it) and you don't want to make those changes publically available, you don't have anything to worry about.
The full text of AGPL is available:
Karl