MacRumors is reporting that Bluetooth LE Adapters do not add Handoff support to older Macs.
If this happens in the final release (which seems likely) it is really shitty of Apple, and just a way of trying to get users who want the feature to buy new Macs.
Especially annoying when something like the IOGEAR Bluetooth 4.0 USB Micro Adapter adds full Bluetooth LE support out of the box without software. So it looks like it is a physical lockout on the part of Apple on older machines instead of a technical issue.
Hopefully if it is the case someone will release a 3rd party hack that enables this very cool feature.