The process has changed a little bit in Mountain Lion for disabling shadows on screen shots, sometimes… By default, there’s no com.apple.screencapture manifest, so the first step is to create it with the boolean disable-shadow key set to true. This part is the same as with Lion:
defaults write com.apple.screencapture disable-shadow -bool TRUE
Now check that disable-shadow shows as a 1:
defaults read com.apple.screencapture
But where it’s a little different is that you previously killed SystemUIServer w/out sudo:
SystemUIServer would then open back up and screenshots wouldn’t have shadows. And this still works sometimes. But now, I’ve noticed across the 30 or so systems in my lab that while you don’t get an error, the process doesn’t actually change the screen shots from time to time. Sometimes, you need to put sudo in front of the killall now:
sudo killall SystemUIServer
Now everything should work as intended.