About 5 hours after signing up for Google Checkout, I noticed a message from Mastercard on the machine stating my card was blocked because of suspicious activity. How could that be? I'm in Canada and there is no way I could have purchased anything using my account even if I tried.
So I called Mastercard back to get more details, and sure enough, the suspicious activity was due to a "purchase" I made in the United States from "Google" (it was only $1.72 or something). I am guessing they put a temporary hold on funds to verify my card, but I don't recall seeing any such warnings during the signup process.
So, if you are having trouble using your credit card after signing up with Google Checkout, you might want to call to see if it's blocked. This charge likely isn't anything to worry about -- but to some credit card companies (maybe only international ones?) it can be flagged as suspicious.
To clarify, Google is not doing anything bad here -- after reading more closely, they clearly state this could happen in their TOS:
You authorize us to confirm that your Payment Instrument is in good standing with the issuing U.S. financial institution, including, but not limited to, by submitting a request for a payment authorization and/or a low dollar credit and/or debit to the Payment Instrument, in accordance with the relevant card association rules. You also authorize us to obtain from time to time a credit report and/or to otherwise make credit or other background inquiries as we deem appropriate to evaluate your registration for or continued use of the Service
This article is simply a heads-up for those who might encounter the same problem. It's not Google causing your credit card to be flagged -- it's the software the credit card company uses to detect suspicious activity. This is completly out of Google's control.