When you look at the baskets Apple has it's eggs in, gaming has probably the fewest. Steve Jobs, if I remember correctly, was not all that excited about gaming and avoided getting Apple invovled in it. But if you're Apple of today, and you want to keep your company growing, you're probably looking at the success of gaming shovelware in your online store and want a bigger chunk. As a bonus, it would give them another area to compete with MSFT in.
That being said, I'm not so sure Valve and Apple would cooperate at this stage. Apple would probably want to start with some kind of exclusive gimmic; at minimum some A+ game available only on the AppStore, or at maximum their own console with its own exclusive launch titles (like Microsoft with the launch of the original Xbox). Valve, by comparison, would not be likely to take on the risk of developing a game solely for a platform that is not yet proven. Valve would likely prefer to have the option of porting their game at leat to PCs, if not to the consoles. Otherwise, Apple could just gamble a huge pot of money to get Valve's (or some other company's) cooperation, or at least their expertise.