hamagc: The RIL is
here, but you've gotta have full Android sources to compile it. It's a big hassle.
First step would probably be to play around with the AT Command Log (instructions
here seem mostly accurate, though i found the log to be written to /ATCmdLog, not /Temp) to see what sort of data comes through when you receive an MMS, and then see how that would be treated by the RIL (you can read the source on Gitorious without having to download it, etc.).