AirDrop unfortunately doesn't work on all systems that Lion supports. However there is a way of getting limited support.

Just open up a shell and type the following:

defaults write com.apple.NetworkBrowser BrowseAllInterfaces 1

killall Finder

This will allow you to AirDrop to other systems that you have an Ethernet or WiFi connection to. While not as useful as the full AirDrop feature, this can still be a handy way to transfer files between systems when you're on the same network.