There is a new simple hack that allows owners of a first gen iPhone to enable MMS on their iPhone 2G. Your iPhone has to be jailbroken to enable MMS. If you don’t know how to jailbreak your iPhone, please read this.
Now go to Cydia and look for the application called ActivateMMS2G from the iSpazio source. Install and reboot your iPhone and bingo!
Very important: this hack only works on OS 3.0 and 3.0.1. Do not install on 2.X or 3.1. Thanks @Stilgar702 for the tip!