Hackers have found that Airdrop can be enabled on any Mac running OS X Lion, and will work over ethernet as well. It can be enabled with a shell command.
Copy and paste the command into the terminal of all macs that are supposed to communicate with other hacked macs.
This will enable AirDrop over both Wi-Fi and wired Ethernet connections:
defaults write com.apple.NetworkBrowser BrowseAllInterfaces 1; killall Finder;
To disable this feature use:
defaults write com.apple.NetworkBrowser BrowseAllInterfaces 0; killall Finder;
This does not enable the feature on your Airport card. The AirDrop feature is originally not there because the Airport card does not support it (simultaneous wifi and ad hoc networking). This hack enables AirDrop to run over whatever other network connection you have. Therefore, in order for two Macs to communciate this way both must apply the hack.