Update: No fix yet, but Google's revised its estimate as to how many users might have been affected by the issue -- "less than 0.08%" -- which means we're probably looking at closer to 150,000 individuals, rather than 500,000. We're assuming that the revised estimate means that the initial count wasn't precise, and not that customers are ditching Gmail in droves.
Update 2: Google's provided promising but terribly vague guidance on when the situation will be resolved: "Google Mail service has already been restored for some users, and we expect a resolution for all users in the near future. Please note this time frame is an estimate and may change."
Update 3: One-third of users have now had their account access restored, according to Google. Read all about it here.
[Thanks to everyone who sent this in]