Tim Cook did NOT visit Valve.
Via Kotaku (http://kotaku.com/5903555/gabe-newell-says-valve+apple-meeting-didnt-happen):
We actually, we all sent mail to each other, going, "Who's Tim Cook meeting with? Is he meeting with you? I'm not meeting with Tim Cook." So we're... it's one of those rumors that was stated so factually that we were actually confused.
Well, time to stop your "zomg apple is buying valve/making a console/getting Half Life 3 as an iOS exclusive" crazy train at the station.