A quick and useful WebmasterWorld informs us that if your credit card is declines by Google, your AdWords quality score will not be hurt.
In fact, AdWordsAdvisor2 replies to the forum post saying:
Being suspended for a credit card decline will not effect your quality score. Your ad will have the same value in the auction upon your return as it did prior to the decline.
It is pretty much like when you pause an AdWords campaign, your quality score should not be affected.
Forum discussion at WebmasterWorld.

Comments:
paul
02/02/2007 04:19 pm
Is this post in response to an error at google? As I have had a problem where both my cards have suddenly become declined?
Kingsley
03/10/2007 11:50 am
My credit card is being decline waht do i do to that it can be accepted by google adword. I need an urgent reply.
reggie
03/28/2008 08:51 am
me too, i am having the same problem up to now. i don't know the reason why my credit card has been declined in google adwords.
jens
05/28/2008 08:29 am
For 2 weeks no adds are running neither - used dozen different credit cards, all declined. WHAT IS WRONG WITH GOOGLE??????? HELP!