Just got my new SanDisk Extreme IV CF card and did a quick comparison (inspired by DPReview's audio file method).
Attached is a comparison of a ~10sec. RAW burst on my Canon 50D. Camera was set to M-mode, 1/1000th shutter and the lens cap was on so all files have the same (black) pixel to make it fair.
Nothing really unexpected here, but it does show that speed of the card does make an appreciable difference with the camera.
Btw. flush times were 7 sec. with the Extreme IV and 10 seconds with the Extreme III respectively.