rythymbox's keyboard interface is incoherent.
The desired behaviour, (which imo is desired behaviour for all music players at all times,) of space bar being play/pause button, is actually possible after you click play or pause with the mouse. It is like it 'selects' that button.
However, after you select a track for example, the spacebar behaviour changes to selecting that track, so everytime you press pause, I mean spacebar, it will again select that track, and because it is playing it will start playing the track again. I hope this will get fixed sometime into something more sensible and intuitive. Like the desired behaviour.