How do I skip the Intro on Binding of Isaac?

All you have to do is hit the space bar.

There is a problem on OS X, which can cause the spacebar to not make the game skip the intro in fullscreen mode:

The problem is that on some Macs the game loses focus during the video when in full screen, therefore the spacebar will not work. Command - Tab to change focus, then reselect the game -- you will be able to use the spacebar then. You can avoid this by running the game in a window.

Source: http://forums.steampowered.com/forums/showthread.php?t=2146137