I've fiddled around with this before. The issue with creating boundaries is that the map is diamond-shaped and not square, so the boundaries would be oddly slanted. This could be annoying if you're for example at the left edge and want to scroll to an area further down and to the left. You wouldn't be able to scroll left and down, because you're already at the left edge, so you'd have to scroll down and then left.
The solution would then be to make a square boundary that encapsulates the whole map, but then you'd still end up with areas in the corners where you go competely off the map into blackness.