[hedphurst]

Does it give you any sort of error message, or is it just acting as if the messages are being sent without actually sending?

Have you tried flashing back to the Alltel stock ROM? This is probably a stupid question, but just to cover all bases, did you have any problems with SMS/MMS when you first got the phone? I have an Alltel-branded Diamond, also, but haven't had any problems with SMS on any of the various ROMs I've flashed from this site. I've had to install Arcsoft MMS on one occasion when a ROM didn't include it, but that doesn't sound anything like whatever problem you're having...