Rockstar has sent us back an email today stating that the PC port of Manhunt 2 is a Direct2Drive exclusive and that the game will only be available in North America. (while the email states that it's "US only", the Direct2Drive page includes US, Mexico and Canada)

Hi AJ,
Manhunt 2 for PC is a Direct2Drive exclusive that will only be available in the US.  There are currently no plans to release the game on Steam.
Cheers

Manhunt 2 for PC will be out on Direct2Drive November 6th.