Here is the post at Travis Oliphant's blog about numba
http://technicaldiscovery.blogspot.com.es/2012/08/numba-and-...
As it is said there, it's still early software and its "road-map is being defined right now by the people involved in the project". Sure the subset is not well defined now and there are no docs, but hey - let's give the thing a few months.
Haven't tested thoroughly yet, but I think no NumPy calls can be made from inside a numba-compiled function - so this might be the case for other Python modules as well.