After hacking around a while trying to resolve this issue, I finally stumbled across a "hack" that seems to get the job done! This was tested on OMJ's custom 6.1 Alltel ROM:
1) Do not let any customizations run after the ROM install.
2) Find a copy of ArcSoft 5.0.31.7 cab [
http://forum.ppcgeeks.com/showthread.php?t=26915 ] and install it
3) When you go to messaging, now it shows "SMS/MMS" and E-Mail, but MMS will not work as-is...
4) Install LennySh Alltel ArcSoft CAB 3.0.26.12c over the top of 5.0.31.7 and soft reset
5) You will now see the same "SMS/MMS" and Email folders, but a "Pics/Videos" folder will now show. This is where you will send/receive MMS messages (you'll just have to overlook the fact that the top folder says MMS, I havent figured out a workaround for that one yet)
6) Make sure your MMS server settings are no gateway, server: mms-mt.alltel.net/servlets/mms, port: 8080, connection: Alltel Wireless (or whatever you have your default internet connection named) and WAP version 2.0
7) Send a MMS message to your handset from another phone. Within 2 minutes you should see the Touch auto-connect and receive! Send works the same as it always did.
I hope this helps, and maybe it will give a clue as to what is wrong with the ROM as-is. I have had a 100% success rate repeating this "hack" several times now after hard resets and re-flashes.