I know that if you buy a PC with XP installed now, it'll come with a Vista upgrade coupon. I don't know if that's specific to certain manufacturers, or to all OEMs.

If it's available to all OEMs, this explains how Newegg are able to get hold of the vouchers. Whether or not you're really supposed to be able to buy an OEM copy of Windows is another point. Ordinarily, you have to buy some hardware (e.g. a mouse or keyboard) to qualify for the OEM license.