So, my phone died again: it stopped recognizing the battery. AT&T sent me a warranty replacement. Of course, it had the default AT&T-branded firmware. I had forgotten how obnoxious it was: everything is locked out, and you have to pay to use them. You can't use the GPS without paying $10/month, you can't play MP3s, you can't use your own MP3s as ringtones, you can't tether the phone to your laptop for Internet access.
The only added feature is that it supports IM: AIM, Yahoo, and MSN. The built-in email client, however, is pretty braindead and doesn't have GMail as an option, and there is no way to manually set the account info.
This is why I reflashed my phone with the generic Sony Ericsson firmware. My phone is actually useful, now. Not bad for $16.
On the other hand, I am not terribly impressed with Sony Ericsson's quality control. This is the second replacement. I've had other SE phones in the past which never had problems, and I tend to use my phones for 3 years or so.