Appeal to popularity, or authority. In both cases, a logical fallacy. The only thing we can glean from this fact is the fact itself: Obama likes Portal (and really, who that's played it doesn't?). It doesn't make Obama any better or worse a politician, nor does it make Portal any better or worse a game.
I'm honestly getting really tired of everybody jumping on something like this - by which I mean celebrities or famous politicians saying they like something. It's essentially meaningless and not really interesting unless you're locked in the narrow frame of mind that says politicians are intrinsically boring people.