Hello Tony,
When I visit a Sandbox URL from a web browser, e.g.
https://sandbox.google.com/api/adwords/v11/CriterionService?wsdl, and
check out its certificate information I'm seeing an expiration date of
March 30, 2011 2:06:52 PM EDT. I'm not having any trouble sending
requests to the Sandbox environment right now, either.
What kind of behavior are you seeing that pointed to an invalid
certificate on the Sandbox server?
Cheers,
-Jeff Posnick, AdWords API Team