My opinion is that there are 3 things that need to be right to get MMS working.
#1 You have to have 3.0 software (beta or GM)
#2 Your carrier file has to be updated to allow (5.0)
#3 Your texting plan can't be for iphone (I have a 1500 Business plan)
To get tethering working would be easier.
#1 You have to have 3.0 software (beta or GM)
#2 Your carrier file has to be updated (I've had it work on 4.9 & 5.0)
This has worked for me without changing any settings, usernames, passwords, sim card switching with other phones, or calling AT&T. I have switched the iPhone off multiple times and sent and received MMS from multiple carriers. Tethering still works to.
As far as MMS goes, I'm on 3.0GM, 5.0 carrier version, and family messaging plan, yet it still hasn't worked for me since Beta 2, no matter what technique I've tried. This blows.