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.
No one here was meeting with Tim Cook or with anybody at Apple that day. I wish we were! We have a long list of things we'd love to see Apple do to support games and gaming better. But no, we didn't meet with Tim Cook. He seems like a smart guy, but I've never actually met him. - Gabe Newell

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.