Long since property of Apple Remote Desktop Enterprise, the ability to remote into your computer without leaving the screen on for all to see has finally shown up, but its in 10.7 only.
Open terminal and run open vnc://yourcomputername
After that is open, click "View" in the menu bar and the click "Switch to Virtual Display". This turns off your iMac screen and allows you to work without watching or playing pranks.
In ARD it was called "Curtain Viewing", I'm very happy to see this finally become a much needed item for everyone