I had issue as once i installed Snow Leopard i was let down to find that Exchange 2003 doesn't not bring in the contacts and events. So i had to upgrade to Exchange 2007 and it was the best thing i done.
Exchange 2007 now works in Sync with iPhone 3GS, Mac and Windows (Outlook 2007). Also it has a nice online OWA.
If your using Exchange 2003 i would recommend upgrading to 2007. Also you can find it on the net monthly for the same price as 2003.