In many ways this is already being done. With Battlefield2, your CD key has to check out before you can play online. It checks everytime you try to play online. PunkBuster does something similar, as it uses your CD key as your identifier.
This is not that big of a deal to me... unless it pops up and says my key is bad some time after I have been playing a while. I would then be very ticked. I mean, Windows did this, and still does with every update.