In OS X 10.1.2 (I never got to look in earlier versions), you can use this line in Terminal to change the format of your screen shot captures when you use Apple-Shift-3 or 4.
This does not affect Grab, which still saves in TIFF no matter what.
If you are one of those that doesn't want to see the command line, there is a cute little utility called Tweaker, and it sets this preference for you. You can download it here, courtesy of VersionTracker:
http://www.versiontracker.com/moreinfo.fcgi?id=12983
Code:
defaults write -g AppleScreenShotFormat [PICT | JPEG | TIFF | PNG]
This does not affect Grab, which still saves in TIFF no matter what.
If you are one of those that doesn't want to see the command line, there is a cute little utility called Tweaker, and it sets this preference for you. You can download it here, courtesy of VersionTracker:
http://www.versiontracker.com/moreinfo.fcgi?id=12983