You can enable teathering and MMS on your iPhone 2G or iPhone 3GS even if you have AT&T or any other carrier by browsing to a website using Safari on your iPhone. Just select your country and carrier and download the profile.
The website is: http://help.benm.at/help.php
This will work WITHOUT jailbreaking your iPhone!
MMS will work for those with a family plan after rebooting the iPhone. Those without are out of luck until AT&T enables it later in the summer.