Right, as far as I understand it, several features are unavailable on T-Mobile pre-paid plans, including proper call forwarding, some voicemail things, and their cheap international roaming. It doesn't really bother me since I only ever give out my GV number. The only calls I get to my "real" number are wrong-numbers.
The only additional caveat I've found is that the proper carrier-level call forwarding to GV voicemail doesn't work with the $30 T-Mobile plan.