updated 02:39 pm EDT, Fri June 20, 2014
Feature so far limited to Macs with Bluetooth LE
Apple is still testing and has yet to finalize which Macs will be able to take advantage of OS X Yosemite's Handoff feature, sources say. With supporting apps and hardware, the technology lets people shift work from an iOS device to a Mac, or vice versa. So far, though, users of the Yosemite beta note that only Macs equipped with Bluetooth LE -- beginning with the 2011 MacBook Air -- appear to be compatible.
A possibility is that older Macs might be able to use Handoff when equipped with a Bluetooth LE adapter. One site, Netkas.org, says it has managed to get Continuity working on Macs as old as a 2008 Mac Pro. Apple could however manually set which Macs will get Handoff, in which case adapters won't matter.
Yosemite's features will likely only be finalized closer to the fall, when the OS is officially scheduled to launch.