Ithappened - I just met a woman who had twins at term, they were 7.5lbs each! So, you're not alone. :) My guess on why twins are born earlier is part Dr preference - less can go wrong if they take them as soon as they are matured, part the simple fact that its harder on your baody to maintain two than it is one. But that's just an educated guess, I haven't really looked into it.
Newmum, Lily, and Keuriweo - Yes, yes and yes! We are also really going to try to keep the stuff to a minimum. No TV/media tie-in's, no plastics, etc. Although its tough to resist some of the cute decor stuff but I'm trying to be strong! I'm hoping that as a family we'll be able to create enough of an active culture that it'll out weigh the stuff they don't have that their friends may have when we get to that stage.