The Tripod Adapter for iPhone 3GS from Gary Fong is a bracket that slides on to an iPhone and mounts directly on to a tripod. It must be noted that the fit it is based on tension, so Gary Fong does not recommend aggressively shaking or subjecting the stand to sustained movement. However, if you're using it in conjunction with their optional Flip-Cage to stabilize vertical video, it's becomes a perfect combination.
Mounts iPhone 3GS directly to tripod
Can be used with an optional FlipCage to create a tabletop stand