One of those things we are all vulnerable to. Even the great Google has let a Cert expire without renewing it properly. Trying to test today in the Google Checkout Sandbox and their SSL is expired and my App is getting errors.
Hope this never happens in production as the Google Checkout Button is dead. One of the reasons for having your own/alternate CC Processors in addition to GoogleCheckout (Matt, are you listening?)
Error: "The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel."