In May 2003, with only a few exceptions, digital system SLRs use sensors that are significantly smaller than a full 35mm frame. 

http://www.dpreview.com/news/0210/02100402sensorsizes.asp

Depending on the camera, the "multiplier effect" means that a 50mm lens acts like a 75mm or 80mm lens.

That's hard to explain to some customers. "You mean my 28-200 zoom isn't 28-200 any longer?" No, that's not quite what I mean. Physically it's the same lens. Nothing in the lens changes. It's more like cropping out one-third of the picture area. On the Canon D60 or 10D, a 30mm lens would provide the same perspective as a 50mm lens does on 35mm film. With the Nikon and Fujifilm models, a 33mm lens is equivalent to 50mm.

(It's nearly as hard as converting APS lenses to 35mm equivalents. Thank goodness APS seems to be on the way out.)

To the best of my knowledge, only three digital SLRs address this by providing imagers as large as a frame of 35mm film: the US$5,000 Kodak Pro14n, the US$8,000 Canon EOS-1D, and the US$7,000 Contax N.

http://digigraphica.com/oped/03/0425.shtml