One of the many new features in OS X Lion is called AirDrop – a new tool that lets you wirelessly beam files to other Lion users nearby without having to be on the same network. Apparently, however, not everyone will be able to take advantage, as it will only run on certain Macs.
Apple has just posted a new KnowledgeBase article explaining which Macs will work with Lion. From the article:
The following list shows the earliest of each Mac model type that is supported. If your Mac is the same, or newer than the model listed, then it supports AirDrop.
MacBookPro (Late 2008 or newer)
MacBook Air (Late 2010 or newer)
MacBook (Late 2008 or newer)
iMac (Early 2009 or newer)
Mac Mini (Mid 2010 or newer)
Mac Pro (Early 2009 with AirPort Extreme card, or Mid 2010)
While its not clear why older machines than these do not support the new AirDrop feature, I suspect that the likely reason is that their wireless hardware simply does not support these types of peer-to-peer connections.