MMS is already available on the 3.0 build. AT&T is simply blocking it on the iPhone while "capacity" issues are corrected. (The same is true for tethering although you can get around that currently as shown in previous posts.)
Basically, there are a ton of iPhones out there right now. I have heard that AT&T split the iPhone data traffic onto systems that are separate of the other devices. All these iPhones push a significant amount of data through AT&T's network- which is why they love for you to use wifi whenever possible . Imagine what happens when you flip the switch and allow all the additional traffic from MMS and tethering.
I personally doubt we will see "official" SMS on AT&T until mid to late august at best. (which still falls in-line with AT&T's earlier statements of it being available by "end of summer.")
From time to time, you have to wonder...... What was I saying?