I have the same problem with my code from Amazon. I just finished emailing playstation.com (customer support) to see what they can say/do. Just wanted to add another voice to this ongoing issue; hopefully Sony will see the number of people with identical issues and get there act together (but i'm won't be holding my breath).
I called the 800 number on the voucher code. Playstation is aware that none of the codes are valid. She said to email the network here: http://playstation.custhelp.com/app/ask and input the contact info, your code#, and a brief description of the problem and the error that you are receiving(for me it was along the lines of "Code is invalid or out of date blah blah blah." Whatever. She said that they would get back to me with an updated code when the solution is solved.
This is pretty sad being that I beat God of War I years ago so I was only buying it for GOW II and the Demo.
I got the God of War Collection the other day, and I was excited for the God of War 3 demo that came with it so I entered the code to download the demo and I am absolutely positive that I am entering it in correctly but no matter what I do it says the code is invalid. It specifically says on the voucher thing that the code is valid until the launch of God of War 3. I really hope that this problem is fixed soon! I would really like to play the demo as I am unbelievably excited for God of War 3! Please fix this ASAP Sony!! A lot of people seem to be having this problem also, so it is OBVIOUSLY something worth looking into!!
Tried the demo code, and it works fine, however the skin code does not seem to work on either the website, or via ps3 by redeem codes option..... this is getting a little annoying & still no word from sony customer support after speaking with them on friday.