From the Google App Status Dashboard:

March 18, 2013 7:17:00 AM PDT

We’re investigating reports of an issue with Google Drive. We will provide more information shortly.

 March 18, 2013 8:10:00 AM PDT

We’re aware of a problem with Google Drive affecting a significant subset of users. The affected users are unable to access Google Drive. We will provide an update by March 18, 2013 9:10:00 AM PDT detailing when we expect to resolve the problem. Please note that this resolution time is an estimate and may change.

March 18, 2013 8:55:00 AM PDT

Google Drive service has already been restored for some users, and we expect a resolution for all users within the next 1 hours. Please note this time frame is an estimate and may change.