Must say that the telecentricity issue, whilst theoretically significant, doesn't seem to be much of a problem at present. If/when sensor pixel counts rise again, it might become so.

As for the size/weight issue - the Canon 40D is very similar in the hand to the 5D, and the Nikon D300 is actually heavier than the 5D - IMHO it would be quite possible to build a full frame camera down to the size and weight of say the Canon 300D. Whether the sort of people who buy FF cameras demand high build standards, or whether there is a yet market for a budget FF camera, are different questions. But, if size & weight were primary factors allowing compromise with image quality, the Olympus three-digit cameras should be dominating the market....

Personally I've always thought that it's the APS-C/DX size sensor that's threatened in the long term - and see no reason whatsoever to change that prediction.