Quote Originally Posted by OldSchool
The first one is lens quality. If you have two 6MP sensors (one full frame and the other a 1.5 crop), one will need a more accurate lens for the 1.5 crop version than the full frame to get the same focus precision. This is because the image is being focused on a smaller area.

BR,
Tim
I'm not so sure about that OS. The cropping effect of the smaller APS sensor actually allows you to get away with a crappy lens because it is at the edges of the lens that the quality of the pro lenses show themselves off. The smaller sensors only capture the center of the image compared to a full frame sensor or 35mm film frame, essentially the "sweet spot" of the lens. That's what accounts for the cropping or magnification factor of the smaller sensors.
Conversely, a full frame sensor will really highlight the edge deficiencies of a lens, even more so than film will show because of the extremely large resolution and quality of these full size sensors. If you're going to splurge on a full frame sensor DSLR, I really believe you need the best lenses you can afford.
Just my two coppers of course, but that's my current understanding of the situation, I could be wrong