When i entered mine, it was rejected. Took me a while to realize my code is an "upgrade" code and not a "complete" code. I had to roll back to XP, upgrade to 64bit vista (yes, that's another lession. 32bit vista can't go straight to 64bit Win 7), then upgrade to 64bit win 7. took me almost a full day to get that thing to work and reinstall all the necessity.
Snow Leopard is the first exception to the rule on this I've seen in forever! IIRC, the $29 SL disks don't check, but all previous upgrade disks DID. ><
What really blows about that is when you stick a new hard drive into an old mac, and have to install the OS twice.