With Apple Remote Desktop (ARD) 3.5.1 (or Screen Sharing!) and Mac OS X Lion 10.7 it's now possible to remote into a Mac without interfering with the current user session.
To set this up...
- From within ARD highlight the Mac you want to control and go to File > Get Info.
- Navigate to the Control & Observe tab and select Edit (lower right corner)
- Choose Connect to a virtual display or Ask me what to do (my favorite). See screen grabs below
The target Mac must be Mac OS X 10.7, but the Mac controlling with ARD can be 10.6 (that's how I tested it).
IMPORTANT: If you connect with a virtual display you MUST remember to log out of your account. If you don't then the user may become confused when they try to Restart or Shut Down and the Mac warns them that another user is logged in (Similar to leaving an account logged in with Fast User Switching).