Far be it for me to anger the gods of connectivity by touting the fact the data functions of my non-AT&T/Cingular phone finally are working, but they are. Here's what it took:

• For Web and MMS, using the AT&T Wireless online configuration tools for mMode (not the Sony Ericsson online configuration tools, those didn't work). You have to be an mMode customer, and cheat as to what model phone you're trying to set up. A woman at Sony Ericsson figured this out. She picked the closest cousin to my phone she could find (the z500) to get the settings.
• For email, finally getting an AT&T/Cingular Customer Care (she typed, without a trace of irony) representative on the phone who recognized I needed to get bumped up to the next level of tech support. Once there, the guy was able to look up my mMode email address (forgotten), and reset my mMode email password (also forgotten, no way to look it up). Armed with that information, I could then use the AT&T/Cingular online configuration tool to send mMode email account settings to my phone. There was one final necessary step before it would work though: the mMode email account settings on the phone needed to be tweaked to tell it to "Connect using mMode GPRS."

I hope this helps some other poor AT&T/Cingular customer who is silly/intrepid/audacious/feature-obsessed enough to buy a phone from outside the silo. Among other things, now I'll be able to use the phone's camera.