I ran GPupdate today on one of my Windows 7 machines and got this. . .
Its kind of a mouthful, but heres the short, sweet story here.
Group Policy relies on the Kerberos protocol. Kerberos relies on the clock. If the clock between your client and your server is skewed by more than the allowable value (normally 5 minutes) then you wont process GPOs correctly !
So, this warning, is saying: My clock is weird versus the domain controllers.
No problem. Usually, a reboot fixes this kind of thing. Or it gets fixed on its own when the time sync service does its thing.
But, one of the key troubleshooting steps for GPOs is to VERIFY that your clients time is within 5 minutes of your DCs times.
Do this, and you’re off and running (sometimes.)
PS: Quick update from Jeff L. who suggested I also turn you on to this Microsoft KB article: http://support.microsoft.com/kb/816042