AirDrop (the useful and efficient file sharing feature in Lion) only works on more recent Macs (from around 2009 onwards?) and appears to be unavailable to many Lion users with earlier Macs.
This worked a treat on my five year old iMac and my four year old Macbook. If it was so easy why were Macs of this age excluded from apple's original Airdrop?
When experimenting with this at the recent Salisbury meeting, we found that although I could see all the other "Airdop enabled" macs in the room , they could not see me. However I appeared when I was sending a file to another Mac, but then disappeared again when the transfer was completed. We could play with this for hours.