for the record: i was only logged in once, in one browser and in one location. i did not reload the gmail page excessively. i did not send a ton of files or use gmail as a filesystem extension.
i DID accidentally drop a file onto safari whilst in the middle of composing an email in gmail, and then hit the back button to get back to my message or to my inbox. gmail locked up, so i waited a few minutes, stopped it, then tried to load my inbox, which was when i got the "anomalous activity" page. while i understand their need to protect against abuse of the service, their paranoia-meter seems to be calibrated a touch on the sensitive side.
on the plus side, my gmail notifier still works, and shows me the header and first couple lines of each message as it comes in via growl. so, if you want to email me today, keep it short and sweet, and if i don't acknowledge you, keep sending it*! i'll eventually see it.
UPDATE: as of 12 noon it's fixed... so I was only sans gmail for an hour or so; not too annoying.