The iPhone 4 needs a case for several reasons. The handset has glass on the front and back making it need a case for protection. The handset also needs a case to keep the user from blocking the antenna.
The other need for a case is to extend the battery life of the device. Exogear has a new iPhone 4 case called the Exolife that has a 1500mAh battery inside. The case adds seven hours of talk time to the normal battery life of the iPhone.
You can pre-order the case right now in black or white colors for right at $90. The case will ship in black on July 30 and in white on September 30.