Lunch break Google Voice experimentation didn't work out all that well. I can call out via GV, but setting up voicemail fails due to the T-Mobile limitations on conditional call forwarding for prepaid plans. My understanding from some googling is that the way to "fix" this is to call T-Mo customer service and tell them to disable voicemail, but I think all that does is prevent the T-Mo voicemail from interfering with your Google Voice voicemail (so the T-Mo one doesn't pick up first) and won't do anything at all for people who call the T-Mobile cell number.

I guess this is just something I'll have to get used to.