How in particular are they blocked by Mongo license? It's AGPL but it only applies to changes made to the database source code itself. It doesn't impose any requirements on your application if you are just talking to Mongo over the wire.
For example, I worked at a public library for years, and the policy was no AGPL, even if we had no intention of touching the source. Same for a local university.
[Edit to add anecdote]
http://www.mongodb.org/display/DOCS/MongoDB+Commercial+Servi...