Though Randy spoke of shutter speed, I think he was using a common term in place of another phenomenon called "rolling shutter" where the camera reads image data off the sensor line-by-line so that by the time its read the last line, the scene has changed considerably since the first line. This is opposed to older CCD-based sensors that read the entire frame off in one shot.
Note that this is *not* "shutter speed" in the normal exposure-time sense.
Not all CMOS sensors have this effect, it depends on the data-output design. In the world of video-enabled DSLRs, the Nikon D90 was one of the worst offenders while my Canon T1i exhibits very little. I understand the higher-end Canons such as the 5D utilize a data-output port that is twice as wide as others in order to reduce the effect in that camera.
You can see it here strongly at 2:19:
http://www.youtube.com/watch?v=O8kvGoCoZkM" onclick="window.open(this.href);return false;
Or even in simple panning:
http://www.youtube.com/watch?v=MeXCRcmOK1A" onclick="window.open(this.href);return false;
Rolling shutter causes some pretty freaky effects with moving airplane propellers:
http://www.youtube.com/watch?v=S1oPiyXu-hY" onclick="window.open(this.href);return false;