Don't submit to the prices at electronics stores! The Speck CandyShell case, which retails at $35, is available on Amazon for considerably less.
The case features a glossy exterior yet a rubberized design, allowing for optimum protection and style. It places rubber atop the volume adjustment buttons (for simultaneous protection and ease of use) and holes for the silence switch and the camera.
It's also available for the same price or just a little more in different colors:
Cases compatible with the iPhone 3GS and 3G also available for around the same prices.