Great article by Glenn Fleishman:
Back in July, I explained how Lion’s new AirDrop feature lets you exchange files simply between two computers with up-to-date Wi-Fi hardware. As I wrote then, AirDrop is a breeze to use if you have the right Mac. You’re out of luck if your computer doesn’t have the right hardware—specifically, if it doesn’t have Wi-Fi chips capable of personal area networking (PAN) for peer-to-peer connections. Many Macs, even many of relatively recent vintage and many that can run Lion, don’t have those chips and so can’t use AirDrop. (Apple provides a list of AirDrop-capable Macs here.)
But, it turns out, there’s a workaround. An anonymous Mac OS X Hints reader found that, if you have one of those older Macs, you can add a setting to AirDrop’s defaults that allows AirDrop to work over regular networks, not just PANs.
Read more at Macworld.com.