A current security feature* is the introduction of leases which are checked by the computer on startup to determine whether it should run. Before a lease expires, it needs to be replaced by another lease, or the machine will stop working.
This is a way of implementing "passive kill" in contrast with "active kill" -- the latter being a way for a central server to be notified of theft, tell individual laptops that they have been stolen and stop functioning, and have the laptops stop working. Both deadly features last until the slain machine is booted with a usb key that reactivates them.
There have been recent discussions of potential use cases for using leases at all; and for theft or recovery scenarios. A major downside of lease use is its ability to render large collections of machines useless without obvious recourse to offline communities, and the implicit transfer of power and ownership away from laptop holders.