Scientific American has some good info on spiral arms:
http://www.sciam.com/article.cfm?id=...tes-and&page=2
I like the bit where it says computer simulations spontaneously generate spiral arms, too, but no-one can figure out why it happens inside the computer, either.
It appears there is not much difference in star density in the arms and in the spaces between the arms, just enough to be visible.