If you bought the Gran Turismo 5 Signature Edition you will undoubtedly using your codes to receive your exclusive cars and enter yourself into a competition, however some people are reporting that their codes are invalid.
I do not own the signature edition, however from what I can see the codes for the stealth and chrome cars work fine, however the code for the exclusive competition entry does not.
I looked into this and it looks as if the codes will become active as soon as the competition starts in December, however at the time of this post no activation date had been announced.
To be honest with you it would be nice to hear something official from Sony as there are a lot of confused people over at the PlayStation forums.
Did your Gran Turismo 5 Signature Edition codes work?