My iPhone 3G was doing very well with the new MMS capabilities until one day it wouldn’t send any. I didn’t update, add any apps, or change any settings. All I did was sync it with iTunes to backup the phone. I don’t know if that caused the problem or not, but it was the only thing I did besides normal usage of the phone. I was hunting for a fix online and saw a lot of suggestions for people who couldn’t get MMS working from the beginning, but none about how to get it working again after it had been working previously.
Some suggestions were drastic including a full restore of the iPhone, and others were annoying by suggesting that you call at&t customer support. All that noise sounded like way too much of a headache to me, so I just did a simple power cycle (turn off the phone, then turn it back on), and that fixed it. Amazingly simple. I’ve made it a habit to start with that as my first step of troubleshooting my iPhone, computers, routers, modems, cable boxes, etc. Sometimes it fixes it, and sometimes it doesn’t. It’s nice when it does.