no it wouldn't (for MS)
people with lesser model will play at lower res, IQ and framerate, that is all.
That would only work if all games were developed to the limits of the basic version and simply ran at a higher resolution on the pro model. In which case you wouldn't get the most out of the pro hardware as resolution is only one part of a games processing requirements.
Unless they treat each version of the consoles as different consoles, but as thuway says that adds even more cost for developers.
It came from a book about the development of the 360.
http://www.amazon.com/dp/0977784215/?tag=neogaf0e-20
Ah right, now I can stop calling it a rumour.
EDIT: Though I don't think much of Dean Takahashi personally, but I assume most of his info comes in interviews with quotes so..