1st Gen Contracts ending
The reason, I believe, that mms will not work on the 1st gen, along with other API's is cause this summer will be the end to many of the 1st gen contracts. Apple will probably release a new phone, with more memory and try to grab those people to renew and get the new phone and enable them to enjoy the perks.
Messaging will open up on ATT for the iphone though, so 1st gen people can jailbreak their phone and use Swirly MMS. A friend told me AT&T would only open mms on the 3G network, but seeing how spotty that service is, it will probably be open on EDGE too.
Just some thoughts.