How can I create a bigger UITabBarItem?

I'd like to re-create something similar to Instagram's tab bar for iPhone, which has a "custom", bigger tabbar item in the middle of the tabbar (see http://cdn.venturebeat.com/wp-开发者_运维百科content/uploads/2010/10/instagram-screenshot.jpg)

I can think of several approaches, but was wondering if anybody has suggestions on an easy way to accomplish this.

check out this post. It explains how its done. http://idevrecipes.com/2010/12/16/raised-center-tab-bar-button/