So we know that AT&T is going to be behind everyone else in allowing MMS and tethering once iPhone 3.0 firmware is released. The question is why. A BoyGenius report via one of their "AT&T Ninjas" fills in the supposed details.
The reason AT&T is going to be slow on getting MMS provisioned for iPhones is laughable. To prevent MMS from working properly on iPhones AT&T has added "Opt Out MMS codes" to iPhone accounts preventing MMS. Once the codes are taken off accounts MMS will work fine on the iPhone with 3.0 firmware at no additional cost -- it is included with the standard $20 SMS/data plan required for iPhones already. The catch is AT&T has to removed these Op Out MMS codes from every iPhone account manually. Hah.
Tethering for AT&T is a different monster. The delay is reported to be completely intentional on AT&T's part and at the launch of iPhone firmware 3.0 tethering will be disabled entirely for AT&T customers. It will become an option at a later date as soon as AT&T has managed to cobble together na unlimited data tethering plan for the iPhone at expense of an additional $70/mo.