More than likely, it's the iPhone-CM7 thing. For some unexplained reason, lots of users here have reported that they do not receive MMS from iPhone users while running a CM7 ROM.
The developers of CM7 for the ASCEND have looked into this & haven't found the cause.
If someone decides to make this their pet-project & find the cause & solution as someone did w/the LED light issue, then we might have a fix sometime down the road.
Otherwise,a fix isn't in the cards,reason being this phone is probably developed as far as it's gonna go,at least w/CM7.
As for CM9(ICE CREAM SANDWICH 4.0) for the ASCEND no one knows at this point.Early reports of ICS stated that it could run on all ANDROID phones,so,a version of CM9 would be the only way we'd ever see ICS on the ASCEND,as both CRICKET & METRO PCS have stopped selling & supporting it,only honoring warranty/replacement service,with no more upgrades to the operating system.
So,in a word,a possible future port of CM9 is probably the only chance of a fix for iPhone-CM7 MMS issues.Same with those experiencing GPS issues.