I have seen lots of different cases for the iPhone over the years. Typically, they offer protection only for your device and nothing like an extra speaker. A company called iMainGo has unveiled a new case that protects your iPhone and has an external speaker for more sound.
The case will fit more than the iPhone with support for the iPod nano, touch, and other MP3 players. The speaker connects to a PMP via a 3.5mm headphone connector. The player can be controlled without having to open the case.
Power comes from four AAA batteries and the case has a removable carry strap. It measures 5.7" x 3.8" x 2.4" and sells for $39.95. You can pick one up right now.