I could definitely see the first part happening but I don't think Cook would go that far for that much.Most likely scenario: Apple is planning on transitioning their Mac line to "retina" displays this year, and when they announce them they're going to want to show off games running at 4K resolution. Valve are one of the only developers who release Mac versions of games day one, so they're an obvious potential partner for such a press event. Cook would be there seeing how well Dota 2/HL3/L4D3/whatever is running on the new hardware and getting feedback on GPU requirements and so forth.
If Cook was telling them about the Retina and asking for their cooperation to begin with, that would ask for a visit, right?
If he just wants to showcase games already done/nearly done in development, right now would give them enough time to the end of the year or something