Sensors are just like cpu's and for the most part, Moore's Law should be fairly applicable. In the electronics industry, 18 months is like 5-7 years in other sectors. There's no way Canon would just sit around and put a 2+ year old sensor back into a new model.
There are a number of "must have" features the new Rebel has got to implement, or else the entire line will be well behind until the next refresh in August 2009. I don't think Canon can wait that long to include spot-metering, more accurate auto-focus, custom white balance, higher ISO range, greater sensor sensitivity, less noise, and higher frame rates.