Ugh... just had my account compromised too. The fuckers stole over $300 from my credit card. They bought 12000 MS points and a Gold Family Pack.
I changed the password on my account and called the bank to cancel the card. The bank said I can file a fraudulent charge claim and I will most likely get the money back. I phoned Xbox live support but they had closed for the night so I sent them an email instead.
The weird thing is that it wasn't even my main account, it was an account I just set up to watch Foxtel on the 360 in the bedroom. The only reason a cc was attached is because you need to have Gold to use Foxtel via Xbox.
I really don't trust MS with my cc anymore.