Do games need antialiasing on the Retina MacBook Pro? Since the pixels in the new display are dense enough that they're imperceptible at a normal distance, doesn't that obviate antialiasing?

(Assuming that the game is running at full 2880x1800 resolution, of course.)
Antialiasing could still play a part in making the rendered scenes look their best, but, yes, the retina display probably obviates most of that. 

As with AA on a non-retina display, it's a tradeoff between the processing resources used and how much better it looks, and that's something only you can judge for yourself. You may find that some AA looks good, but that more-intensive AA isn't necessary.