[font color=\'#000000\']Interesting idea. Nikon kinda did this already with their D1H and D1X, didn't they? Fast, lower-megapixel sports/journalism model and a slower, high-megapixel model. I don't remember the price points, so maybe that isn't a perfect example. And they are totally different cameras - you can't buy one camera and switch between "X" mode and "H" mode.
I would be surprised, however, (happy, too, but definitely surprised) to see the day when most people are content to keep a DSLR body for a decade or 2. I'm amazed there is any interest in going from the Canon 10D to the 20D, for instance.[/font]