Some of the press activity and 3.0 talks mention the experimental in-memory storage engine but there is no info in docs on how to test this out.
The current implication is that mmapv1 and wiredTiger are the only valid storage options: http://docs.mongodb.org/manual/reference/configuration-options/#storage.engine.
We should add a note with appropriate caveats on usage:
An experimental in-memory storage engine is included in MongoDB 3.0 for development & testing purposes. You can try this out with a storage.engine value of "inMemoryExperiment".