AT&T has MMS blocked on all iPhone accounts. There are three things you can do:
1 - Switch your SIM card into another phone for a few hours, use normally. Send txts, make calls, try sending MMS, etc. Eventually you should be able to send MMS messages. If not, go onto your account on AT&T's website and change your phone, messaging plan and data to a non-iPhone, a non-iPhone messaging plan, and a non-iPhone [unlimited] data plan. Continue to make calls, send txt/mms, etc.
----After you are able to send MMS, swap the SIM card back into your iPhone, it should work.
2 - Call AT&T and try to have them remove the MMS opt-out from your account. Tell them you lost your iPhone or broke it or something. Have them switch you to a non-iPhone messaging and data plan.
3 - Go back to carrier file 4.0 and wait it out. It's only three weeks away, at most. Rumors around here (remember, they are rumors only) are that it will be released at the Apple event on September 9th.