Not sure if this happened to anyone else, but when I tried launching the game via Steam I just got the launcher and then nothing.
Running the stand-alone version from the Humble Bundle actually gave me an error about steam_api.dll being missing. I just grabbed the one from TF2 and stuck it in, works fine now.
Copy steam_api.dll from ...\Steam\steamapps\common\Team Fortress 2\bin to ...\Steam\steamapps\common\Mnemonic
It possible to update the Steam build to include the DLL, or are my PATH variables just busted?