Finally got it working...
Here's the sequence of events that worked for me.
1) I installed the modified ipcc file.
2) Put my SIM in my BB and went to the AT&T media net website discussed
here, entered the four digit code that was texted to me.
3) I went to my AT&T account settings online and changed from an unlimited iPhone messaging plan to an unlimited "regular" messaging plan.
4) I made a call from the BB and sent sent a couple SMSs and MMSs, just for kicks.
5) I put the SIM back into my iphone. After several minutes I found the iPhone was able to receive MMSs but not send them.
6)I called AT&T and first spoke with a representative that said they had just received an email regarding MMS and the iPhone and that the service wasn't ready to be activated yet. I insisted that some developer friends of mine have spoken with tech support and had it enabled, and that as a developer I needed to be able to test MMS functionality. He put me on hold, spoke with tech support himself, and came back and told me it had been provisioned, but that I would have to call apple to get the carrier updates to make it work.

(That was my step 1)
I tested it and it works fine. Let's see how long it works...
Now that we've done a bunch of tweaking to phone and AT&T account settings, I'm also curious what we're all going to do when MMS function is
actually turned on in whatever beta of 3.0 that will happen. I'm guessing there will be another call to AT&T in my future.
