You know the one that has your “Phone”, “Mail”, “Safari”, and “IPod”.
Well you can change these around or replace them with other Icons entirely. In order to do this just press and hold the icons. When the icons start shaking, just drag them while where you want them. You can put any different 4 icons on your dock at a time. Shown to the right side of the screen is a screenshot of how I changed my “Phone”, and “Mail” and reversed them.