OS X Yosemite’s Handoff Will Require Bluetooth 4.0
Handoff will be one of the coolest features of OS X Yosemite and iOS 8, letting you seamlessly transition between workflows on the two platforms, but unfortunately for owners of older Macs, it will require Bluetooth 4.0, which excludes Macs built before 2011, and even many built later. Peter Cohen of iMore explains how to check which version of Bluetooth your Mac has. If your Mac isn’t compatible with Handoff, a USB dongle that provides Bluetooth 4.0 may enable Handoff by Yosemite’s release.
Not totally clear yet that a USB Bluetooth 4 dongle will work - see the comments to Peter's piece. I hope it will, as all of our machines are pre-BT 4.0 Macbook Pros, recently upgraded with extra RAM and SSDs - no plans to get new machines, but would hate to miss out on this feature.