Petter Silfver gives us 5 rules for using the iPhone tab bar
The tab bar is the one thing that is constantly going to be the face of your application to guide the users as they navigate your application. Don’t underestimate the time you need to put into creating a flawless tab bar experience.
Some great lessons about using the tab bar. A pet peeve of mine is the "more" tab. If you need more than 5 tabs please use a "basement" instead.