Glad it helped. Here is a great read on z-index. Many folks don’t really understand how it plays out and get confused when an element with a higher z-index value doesn’t always override an element with a lower z-index value. It’s because of the browsers cascade ordering. Once you read this you’ll be top dog on this subject and can rule the quad during the office break time debates =)
oh and ps, I think that is a default setting on the columns so nothing that wouldn’t have been a bugger for anyone with a fixed nav bar. Personally, I didn’t even notice it until I re sized the height of my browser window. Tricky things these interwebs.