T-Mobile could work but at Edge (2G) speeds only. Their "4G" is incompatible although they are starting to roll out 1900MHz service (which could be compatible) but I have no idea how widespread that is.
T-mobile doesn't have LTE yet so no word on that until 2013. Also 3G is incompatible with both the AT&T and Verizon versions of the iPad. The 2012 iPad will work on T-mobile with EDGE speed which isn't too shabby.