CMOS - Complimentary Metal Oxide Semi-Conductor, as opposed to the alternative - CCD - Charged Coupled Device.

CCD's require much more power, and generally don't produce as wide dynamic range, and comparatively speaking produce a lot more in-camera electrical noise (largely created by much higher voltage input). CMOS runs off of much less power, thus extending the shutter-cycle life of the camera, produces better image performance on noise and color, etc.

I've been saying it for years, CMOS is the future, CCD is just biding its time until its history. Probably something like APS-sensors ;).

All the cameras you mentioned have CMOS, another Camera you might find worth looking at is the Sony A700, its an APS CMOS as well, its what I shoot with, I am thrilled with its performance, and its right at that price point you are looking at. But ultimately, you know you are not only just buying a camera, you are buying into a system, there are a lot of variables worth considering. Keep in mind that what camera you are shooting with makes *no* difference to any client, *any* DSLR system can produce great results that even the pickiest of clients would love - the difference it makes is exclusively for the photographer. When someone gets a painting, they don't care what brand the brush was, the type of brush matters only to the painter.