This project includes a copy of the custom tab bar that I developed for the Holler iPhone application (pictured below).
While there are a number of ways to create custom tab bars, I found that creatting a custom object was the easiest way for me to accomplish this in a short amount of time.
In under an hour you can essesentially have a custom tab bar of any design. The key files are included in the /Custom Tab Bar folder within the XCode project.
Keep in mind that this is not a framework for custom tab bars, just the code that I used. You'll have to modify the files to make things work.