Hidden behind the white goop is three images (so far). One of a broken, smoky hospital; another of an empty waiting area, also in a hospital; and a doctor’s office.
It’s probably hinting at the long-awaited Meet the Medic video, but Valve is like the Machine of Death of trickery. You can investigate more at the official Team Fortress 2 blog.
Vic: I’d also like to add that 78 static birds (doves) have been added to 21 of the TF2 maps. Shooting or touching these will make them explode, presumably in a shower of blood and lovely feathers. There’s also flying doves, that appear from the corpses or gibs of a killed player, usually Scouts. Doves have also appeared in a leaked Meet the Medic screenshot.