My OS X screenshots with both cmd-shift-3 and cmd-shift-4 have always defaulted to PNG out of the box. I've been on board since Tiger and on two different machines.
I really love Skitch for taking screenshots.<p>You can go from snapping a shot to having it on the web and pasting a URL to a field in like 5 seconds. Like such:<p><a href="http://skitch.com/bprater/xri2/hacker-news-mac-101-capture-sharp-screenshots" rel="nofollow">http://skitch.com/bprater/xri2/hacker-news-mac-101-capture-s...</a>
My guess? The guy took a full-screen screenshot, opened it up in Preview in Auto Size mode, and was surprised that it looked fuzzy when shrunk to fit on his display.