Millenium on 14 December 2011
This decision is rather simple for MS & Remedy:
Remedy can make more money out of it.
Microsoft had it's exclusivity and can make more money out of it.
The game was initially being developed for pc, so the content & engine should be pretty port ready.
Mainly of all, Alan Wake isn't huge like Gears of War or Halo which they will most likely keep for the 360, if Alan Wake had sold ~5 ~6M on the 360, it probably wouldn't have gone to the pc.
Also Microsoft is a company, companies like Money, and the EED has been having to put it's foot forward to Ballmer & Co lately in terms of financials, this will help to do that even more.
