Well, this is certainly out of left field: the latest Team Fortress 2 patch has one of the Engineer's new weapons and a secret message. It's a good chance that Engie update isn't too far off from now.
Curious file divers took a close look into TF2's patch files and found a working Golden Wrench that turns corpses into gold. While it has no other known effects, it looks like ol' hard hat's unlockables dabble a bit into alchemy; there's also the new shotgun with the little doodads seen in the TF2 Mac debut trailer.
The hidden message left by Valve came in the form of garbled text within a nearly-blank image. After being decrypted, the text hints at something else to be found in the imag that hasn't been discovered yet. Once more, users at the Steam forums have gathered to figure out the mystery.
Update: Just a few moments ago, a new TF2 comic involving the BLU Engineer teases what may be in store. Read up on it and keep an eye out for some hidden links.