Open Terminal and type:
defaults write com.apple.ScreenSharing DoNotSendSystemKeys -bool YES
To restore the old behavior, change the last word to "NO." You can also use a plist editor if you're scared of the Terminal.
If you prefer the 10.5 behavior, wherein special keys are never sent to the remote Mac, there is a solution. It requires Terminal (in Applications -> Utilities), but only for one command. Launch Terminal, then copy and paste the following line (while Screen Sharing is not active):[From Don't send special keys in 10.6's Screen Sharing | Mac OS X | Mac OS X Hints | Macworld]