Before Lion, your Apple ID could only be used online, in the Mac App Store, or iTunes Store. But now, Apple allows you to link your Apple ID with your user account in OS X Lion. This enables you to do things like sign into screen sharing with your Apple ID and use your account with Air Drop and as authentication for File Sharing.
The Screen Sharing application has been around in OS X since the days of Leopard (oh so long ago!), but Apple has added some nifty features to the application and underlying support in Lion. From a new tool bar, to a new per-user screen sharing feature, Lion has you covered when doing screen sharing on your local network between two or more Macs.