Manufacturers have been using these sorts of "disable" techniques to deliver a product to different segments of the market for a while (example, locked CPU multipliers) and it's fairly known that AMD's Phenom triple cores are really a quad core with the 4th core disabled.

A bit old, but I happened to read that the 4th core of the Phenom II X3s can actually be quite easily unlocked.  Now one thing to consider is that the 4th core may be disabled for a reason, ie defective core, but if you're lucky, you could get a quad core Phenom II (which are already good value IMO) for the price of a tri core.

Apparently the mobo needs to support Advanced Clock Calibration and be some 790X-ish chipset.  The ACC needs to be set to Auto or something.  Anyway, if you're interested do a search on Google for various articles - this is just a heads up if anyone's interested.