What direction is north in the editor?

North is whatever direction you want it to be. The world coordinates don't have anything to do with real life and can't be translated to real-life directions. In my game, negative Z might be north, but in your game, maybe positive Z is north. Or negative X. Doesn't matter really, as long as you stick to whatever convention you create.

Edit: Now, if you want to get picky about it, you could say that looking from the top down on your scene, positive Z is north, negative Z is south, positive X is east, and negative X is west. But really, that'd be more of the default assumption, not a rule or a fact. In reality, those four directions would be just up, down, left, and right, which as you know also don't directly equate themselves to any particular compass directions, they're just relative terms.

Short answer: Orient your world any way you want to, as long as you remember to keep it consistent. :)

The upper-right corner of the Scene view has a manipulator indicating your current view. You can click on any of the coloured axis pointers to look in that direction. As Jason says, north is where you want it, but the view widget can help you get reoriented to the coordinate system. Click on the Y axis (green) for a top-down view.