With Vista, if you replaced hardware on your machine or put your hard drive in a new computer, Windows would prompt you to activate online. You were typically given 5 or so activations before your key would no longer work.
With Windows 7, if you replace hardware, you still get the Windows Activation message saying that you must activate in 3 days. However, now you must reactivate using the automated phone system. Online activation can only be accomplished if you are reactivating with the same hardware.
I wonder if you can now reactivate more than 5 times on the same hardware? This would make sense since they are storing your hardware information to match up with your activation key.