Received from Google via e-mail to a non-Gmail account:
Since about 2 p.m. Pacific Time today, many Gmail users have been
unable to access their email. We are very sorry for this interruption in service. The issue is being caused by a temporary outage in the contacts system used by Gmail which is preventing Gmail from loading properly. We are starting to roll out a fix now and hope to have the problem resolved as quickly as possible. Even though you may not be able to get to your inbox right now, your mail is safe, including new incoming messages.
Further background is available from Rafe Needleman at WebWare.