3GS Jailbroken was using 5.1 carrier file on 3.0 firmware.
i installed 5.5 att file and MMS was not working...i rebooted the phone still nothing..
here how i fixed it...
went to Settings > General > Reset (at the bottom) > reset Network settings
then reboot (it may do so by itself)
after that my MMS started working, slow but it worked...
Central Jersey
Did the same thing, still doesn't work