Hi obay, My understanding is that it is added as a credit to your account. I'm pretty sure they do it just to verify that a legitimate credit card has been used.
Unfortunately, it is not applied as a credit. I think they charge it to fatten their bottom line. https://adwords.google.com/support/bin/answer.py?answer=6296
It's a account opening fee and google will not gives those money back but yahoo search do give back $30 account opening fee back to account as credit