Okay... update... just got off the phone with AT&T... the first customer support person tried the previously mentioned fixes, they didn't work, and they forwarded me on directly to tech support. I talked with a really helpful guy who just reconfirmed all that we'd done... and we found the culprit. if you hit #*06# on your keypad, it gives you some sort of internal AT&T routing ID (forgive me for my ignorance...) anyhoo, that number didn't match up with the number that AT&T had in their system... the number in AT&T's system was that of a 2G iPhone. Like I was guessing before, I'm pretty sure this is because I first opened my AT&T account with a first gen (2G) iPhone, and then in March upgraded to the 3G. I guess the AT&T system never updated properly...
Anyway, the tech guy escalated my case to the west region tech office, and I'm supposed to get a call back/resolution by the 30th at the latest. (although he said that 24-48 hours was more likely) This isn't an isolated case, as he also mentioned that he'd handled 12 of the same problems in the past 4 hours...
So, in summary, if you already have MMS working fantastic... If not, and you're getting the 90% and then failure thing, give AT&T a call & best of luck...