Try using some nail polish remover. It worked when I got permanent marker on my iPod. Try cleaning a little area first to make sure that it won't damage the MB first. If it is okay, try cleaning the rest of the marks. BE CAREFUL, use a Q-tip to get the pen marks off. Remember, Nail polish remover REMOVES nail polish so it might remove more than you want to.
Like I said just BE CAREFUL. Take a Q-tip and dip it in the nail polish remover and squeeze out any access then gently swab the pen marks. I know it sounds crazy (you're probably saying to your self, "what is this guy thinking), but trust me, if it can get rid of permanent marker it can hopefully get rid of the pen marks.
Prof.
MacBook Core 2 Duo 2.16GHz 1GB RAM 120GB HD Mac OS X (10.4.10) 30GB White iPod w/ Video Gen. 5///4GB iPod nano Product (RED)