Hey, I just experienced something really annoying and I thought maybe I can warn some people about it.
So I live in Germany, I usually buy my retail-versions of PC games in the UK since they are (even with shipping) cheaper there. I did that with Darkness 2 and the game just arrived. When I tried to install it and it asked me to put the serial into steam, steam told me this version of the game is not available in my country. Germany usually has low violence versions of games unlike the UK but that has never been a problem for me. I also checked the amazon.uk description of the game and didnt find any warning that this version might not be working in a different country.
As of now I found no way to work around this, and this is very specific to the german-uk think, but everyone who imports the game on PC should probably be carefull.