Last night, at 6:31 pm Pacific time, he tweeted a photo with the caption, "About time to unveil the D and something else".
The photo showed a roller door with the letter "D" on it, the shadowy front end of what appeared to be a Tesla Model S, the date October 9, and the Tesla logo.
Then the Internet exploded.
Eight hours later, the cryptic message had been retweeted 7,850 times (and favorited another 5,800), and acquired a lengthy list of responses and comments.
Many of the comments suggested that "D" stood for something other than what than Musk may have intended. Hilarity ensued.
Our guess: Since the photo appears to show a Model S, we suspect that the October 9th announcement may involve updates and new options for that electric luxury sedan.
It's long been expected that Tesla will offer an all-wheel drive option for the car--derived from its work on the upcoming Model X crossover utility vehicle--and perhaps this is the time.
Recently, the Model S also quietly acquired some new electronic safety systems, by the way: adaptive cruise control and lane-departure warning.
Those systems require additional sensor hardware, so they can't simply be retrofitted to existing cars via Tesla's "over the air" software updates.
It's also possible that the announcement will involve a new, larger battery pack option for the Model S. Owner forums have discussed a rumored 110-kilowatt-hour pack for several months now.
Like everyone else, we don't know for sure what will be revealed on October 9, though we'll cover it when we find out.
But we don't expect it to be the Model X crossover, which is now scheduled to enter volume production next spring.
The production version of that vehicle is still pretty much missing in action.
Meanwhile, Musk seems to be enjoying himself.
Shortly after his cryptic "D" message, he tweeted, "I love the Internet. Comments had me literally ROFL. No, it wasn't intentional. Glad I didn't mention the other letter!"