Microsoft's Nick White posted an update on yesterday's WGA server outage

Earlier today (25 August) we began receiving reports of Windows Vista customers running genuine systems but experiencing failures when attempting a Windows Genuine Advantage validation. This situation has since been resolved and validations are now being processed successfully.

We've since learned that very few customers were affected. Sr. Product Manager Alex Kochis and a team of engineers have been working on the issue and has blogged about it to keep customers up-to-date. Customers who received an incorrect validation response can fix their system by revalidating. I'd encourage anyone who received a validation failure since Friday evening to visit this site now; after successfully revalidating, any affected system should be rebooted to ensure that genuine-only features are restored.

Addressing WGA validation issues