FYI, in case anyone's run into this:
We've been receiving reports on our forum and through customer service starting last night that Windows Vista validations have been failing on genuine systems. It looks now as though the issue has been resolved and validations are being processed successfully.
Customers who received an incorrect validation response can fix their system by revalidating on our site (http://www.microsoft.com/genuine). We encourage anyone who received a validation failure since Friday evening to do this now. After successfully revalidating any affected system should be rebooted to ensure the genuine-only features are restored.