Hey, I hope I've reached you in time, but I have used a great script that is not only phenomenal (in my humble opinion) but also free! It can rotate banners just like you want, but it can do much more than that. It can rotate any image, in fact, it will even rotate html code. In other words, you can use it to rotate virtually anything, including complex ad code!
You can find more information and the free download at:
http://www.focalmedia.net/htmlrotate.html
I advise anyone looking for a similar solution to check it out! I hope this helps.
Thanks and Good Luck!
Benjamin
P.S. Why not consider joining my banner exchange? The link is in my signature.