This Saturday, on June 13th, between 10am to 2pm (PST), Google will be performing system wide maintenance on the AdWords platform.
The Inside AdWords Blog wrote:
On Saturday, June 13th, 2009, the AdWords system will be unavailable from approximately 10 a.m. to 2 p.m. PDT due to system maintenance. While you won't be able to sign in to your accounts during this time, your campaigns will continue to run as usual. We apologize for any inconvenience.
This is typical monthly maintenance, so no need for concern.