AppleInsider details changes to Screen Sharing in Lion, including this fantastic bit:

In Mac OS X 10.7 Lion, the built-in Screen Sharing feature, updated from 1.1.1 to version 1.3, now allows remote users to log into a separate user account from the one that is currently logged in. That means that while one user is logged into a machine, a second user can login to the same machine remotely, seeing their own desktop and user environment.

This sounds fantastic.