Answer: Most likely.
Valleywag first heard the news when a keynote speaker at a Vancouver-area "Facebook Developer Garage" event on Tuesday had to cancel in advance because he learned that he had to be present for a company all-hands meeting that day. The gossip blog promptly speculated that something rather big might be on the way very soon.
To do some investigation, I promptly checked my Facebook friends list to see if any company employees who have kindly "friended" me had anything incriminating in their "status" messages. I won't quote them directly, nor will I disclose who they are; Facebook profiles are not technically public so I consider information on them to be background rather than on-the-record. But here's what I found:
One Facebook employee who is not based in the Bay Area had a status message that said he was en route to San Francisco as of Sunday, and one internal PR rep's status hinted at an impending influx of e-mails before changing it to something implying a crazy week ahead that would likely result in sleep deprivation. The latter could simply mean a hellish load of meetings, but the former (which was soon deleted and erased from the "mini-feed") indicates that there'll be something going on at Facebook that us nosy journalists want to hear about.
Later on Sunday, a Valleywag blogger added some more information: "Valleywag now hears that Facebook has a sales conference set for Tuesday, and, possibly, a tech-department all-hands, but a companywide all-hands is unconfirmed."
Cool, but why are publicists bracing for the storm?