The Mongoid guys are working on the 3.0 release now it seems, and mongoid-rspec doesn't support it out of the box as things are now.
I've created a fork with a mongoid_3_0 branch where I'll commit any problems I find when implementing Mongoid 3.0 in an application I'm working on right now.
This is probably by no means complete, and shouldn't be pulled until 3.0 is ready anyway - but its a start!
Changing version dependency on mongoid to >= 2.0
Using default_val instead of default if running Mongoid 3.0 or above
prefere the #61
Added support for newer versions of mongoid with #61