in M mode, rotating the quick control dial (the 'dial on the rear') adjusts the aperture. For HDR, you need to keep the aperture constant to have the same DoF in all your shots. So, in M mode, rotate the main dial (the one behind the shutter button) to adjust shutter speed.
Not on my 7D, it was the first custom function i changed. In Tv the dial by the shutter changes time, in Av and M the dial does aperture, and in M the wheel on the back does time. Can a 60D or lower do this? (real question, not rhetorical).
it doesn't take much to just add it to other bodies.
No, it doesn't. But a 1Ds3 is 2-3x the price of a 5D2 with the same sensor. They have to justify the price differential somehow, like using better AF, built like a tank, more 'little' features. This is one of those nice 'little' features, not worth a grand on its own, but they all add up...