I don't actually have any links for you, unfortunately. This was gleaned from talking to a Maemo developer very familiar with the Linux kernel who explained it to me. MMS is nowhere near as simple as people think... it's like a separate connection which needs to cut off the current one to work... at least in the kernel in the N900 now from what I understand. Upstream has apparently committed stuff that should allow it to work now. So it's not a matter of something that was forgotten, but something that just couldn't be implemented at the time.