John Carter asks, "Do you have Lion installed on your Mac? Have you tried to use AirDrop yet? Are you not even sure what’s its good for? Get a real good explanation about AirDrop here.
"Note: AirDrop is only useful if you are attempting to collaborate with other Mac users in an environment where Internet access is not available.
"As explained by the article in the above link, not all Apple computers running Lion (10.7) support AirDrop. It appears that a Core 2 Duo machine does not. My iMac is a Core 2 Duo. Bummer!
John goes on to explain, "I confirmed this on my own iMac. My MacBook Pro clearly showed AirDrop in the Finder, but on the iMac it was not there, and it also did not appear in the Finder->Preferences-Sidebar window. Here’s what you should see:

"But do not despair, there is a way to enable Airdrop on any Lion machine. The instructions are simple, but you do have to use Terminal and enter a couple of commands, which the instructions make perfectly clear.
"Even after I enabled AirDrop on my iMac which then showed my MacBook Pro in AirDrop on the iMac, when I went to my MacBook Pro and looked at AirDrop, the iMac wasn’t there. So, I decided to do the same two commands on the MacBook Pro and — voila! — the MacBook Pro showed up. I tested transferring a file between the two machines and it worked as advertised.
"So despite the fact that my iMac supposedly does not have the right hardware, the 'fix' made it possible to use AirDrop anyway."
John Carter wants us to know that Prez Art Gorski was co-author on the above. Thanks to both of you!