Same. All they said to do was run Steam as an administrator and verify the Game Cache, then washed their hands of it and said talk to Introversion.

I'd like to say that I am literally in the EXACT position as the poster above - except that there were three of us that bought it at the same time and it's refused to work for all of us, telling us that authentication failed. Have tried everything - manual deletion, deletion through Steam and reinst...