They just tweeted 'Start spreading the news, I'm leaving today.'
Duke Nukem: Manhattan Project? :)

My guess : 3D Realms / Rockstar or some sort of alliance with Steam.

Damn, missed that one. :)

id Software

I'd be very surprised if it was Manhattan project. According to wikipedia (not the most reliable of sources in the best of times, but that's all we've got for now) the game is tied up in rights hell because of a combination of lapses and negligence.

Oh yes! i'll also go with Sierra since i really want the Swat 4 gold edition here on GOG, i only have the original Swat 4 minus the expansion.

Not that my pick is necessarily "right" (Outcast), but would the reveal of either iD, 3DRealms, and/or Apogee really induce "illicit tears of jubilation from within Tom's soul"?
I think it would have to be something a lot more illusive then those studios.

I'm thinking IiD as well :)
Go iD

I'll go with iD. Whichever developer it is, I probably won't be buying their game/games (not much of a fan of shooters), but a free Spellforce would be great. :)

3D Realms.

Im also gunna go with 3D Realms, because for some reason i have Rise of the Triad in me head =/

I just tweeted @ GOG that the last hint sealed my entry: it's Atari.
BJ

Infogrames! Bring me independence war GOG!!

That would make lotsa peeps happy, my copies of Baldurs Gate and Icewind Dale have atari logos on...
Would open up a lot of possibilites.