Spoilers ahead.
The official Skyfall twitter feed last night shared this image:
It doesn't take too much knowledge of military protocol to decipher a hint of something there, but if you want it made more explicit, there's more info to be had from on-set snoopers.
Footprints of London went walkabout in Greenwich yesterday, and scored a peep at the film unit. Here's their picture, which underlines the suggestion of the official Twitpic pretty clearly:

There are more details in their written report, which crucially reveals one character who won't be found lying in one of these coffins:Eight coffins draped in union jack flags and arranged in two neat rows occupied the vaulted ceilinged room which normally houses the elegant tables and chairs of the King William Undercroft restaurant.
MI6 chief M, played for the seventh time by national treasure Judi Dench, was filmed in the undercroft, surveying the gruesome aftermath of a mass assassination of her agents. Thankfully, although entirely predictably, James Bond ? portrayed for the third time by Daniel Craig ? was not among them.
We've been hearing that there's a change of M coming, with Judi Dench giving way to Ralph Fiennes. At least we know she's not going down in this particular massacre.
Eight agents, all taken down at once? Could this be the Skyfall of the title? It has to mean something. Apparently.
At time of writing, we're 292 days away from Skyfall's release.