A more precise way to identify Bluetooth 4.0 support is to select "About This Mac" from the Apple menu, click on the "More Info..." button, then on "System Report." Select "Bluetooth" under "Hardware" in the left-hand pane, and then find the item "LMP Version." LMP stands for the "Link Management Protocol" used to set up and manage a Bluetooth connection between two devices.
If the value to the right of "LMP Version" is "0x6" then the Mac supports Bluetooth 4.0 and so can use Handoff when the system is upgraded to Yosemite.
OS X 10.10, aka Yosemite, will run on a majority of current Macs — by Computerworld's estimate, about 80% of those that went online in May — but the Bluetooth LE mandate means that not all Yosemite-powered machines will be able to hand off tasks or accept incoming jobs from other Apple devices.
At a minimum, 25 million Macs will support Handoff — that's the total global sales from the fourth quarter of 2012 through the first quarter of 2014 — but the number is certainly somewhat higher, since the 25 million doesn't account for five quarters of MacBook Air sales, one quarter of MacBook Pro sales, five quarters of Mac Mini sales, and whatever number Apple has managed to sell of the redesigned Mac Pro.
At WWDC, Apple CEO Tim Cook said that the Mac installed base was approximately 80 million machines, which means that at least 32% of all Macs in service will be able to use Handoff.
It's not clear whether third-party Bluetooth adapters, such as the inexpensive dongles that plug into a USB port, will allow Handoff on older Macs.
OS X Yosemite will launch later this year — most likely in October — as a free upgrade.
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