I'm not sure whether or not this happens in Tiger, but it seems that if you mess with resolution independence (the AppleDisplayScaleFactor value), then OS X's built-in screenshot tool stops working.
The solution is to open up the .GlobalPreferences.plist file in ~/Library/Preferences and delete the AppleDisplayScaleFactor string, save the file, and log out and back in.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071203185154565