I've never read anywhere, on any forum that IS adds noise to an image. With all the IS lenses I've owned I've never seen IS have any effect on noise. The noise in that image of Scott's is so close any difference is undecernable in my mind. You take 2 diffeent shots even with manual setting you could have a small difference in noise like the above.
Older versions of Canon's IS (28-135, 100-400L) require you switch it off on a tripod, everything with generation 2 and up (including the 70-200 f2.8L IS) auto sense on a tripod and switch IS off automatically. There have even been arguments about that. There was a fairly long thread on FM a couple of years ago arguing that IS must be switched off on a tripod even with the newer versions (regardless of what Canon claimed) I posted shots taken with my 300 F2.8L IS of a pod with both IS on and off, (that were absolutely pixel to pixel identical) along with a few others and that pretty much ended that argument.
2 detriments to IS: added cost and added bulk.
If you want to see a difference in noise levels compare images from any Nikon to Canon DSLR from ISO 800 and up (and even that difference is not really that big of a deal.)
Now here is a difference in noise. BTW the noise in that shot is so bad it took 4 passes from noise ninja to smooth it out at all and then PS blocked it up anyway when I resampled it.
(Message edited by richd on March 02, 2007)