If you bought one of the updated models from June 2012, don't forget you can get the upgrade for free: http://www.apple.com/osx/uptodate/
Interestingly enough I went through their redeem process (which is sort of convoluted, with a password protected PDF), and was then told "This code has already been used. Each code may only be used once." when I tried to redeem the code they gave me... So you might want to wait a bit until they sort out the bugs.
edited to add: I've now had a response from apple after mailing support. We're getting new codes. NB they do mention doing this in response to a claim, so it appears you did need to contact Apple if you had a problem!
"Before I proceed I would like to point out that Apple is aware of the issue and are working hard to resolve it ASAP...If your claim is accepted, a valid replacement code will be sent to you by email. There will be no need to submit a new claim....Please let me know if you do not receive a replacement code within 24-48 hours, as I'll be happy to look into things further for you."