After activation it was hosed again. There's a RIM help page that says if you upgraded you have to make sure the two T-Mo certificates are installed. This implies that without the certs, you'll get no UMA connection whatsoever, and I'm getting intermittent performance (and the certs are there). So something else was wrong, and it was coming from my BES server. I checked Options -> Advanced -> TCP and found APN blank. So I put in "wap.voicestream.com", left the user and pass blank, and pulled the battery. VOILA!

I've attached bandwidth profiles for a good UMA call (before BES activation, alternating sending and receiving as one party or the other talks, consistent speeds) and a bad UMA call (both sending and receiving are dropping out every few seconds). After I fixed my APN, pulled the battery (just for laughs) and tried again, my UMA call looked like the "before activation" picture.

Well THAT is a relief.