There are some tripod mounts available for purchase: Joby makes one, and I also just discovered the Glif, a product designed by two guys and launched using KickStarter. I love how versatile and simple the Glif is, but I'm sad it won't accomodate a protective case like the Otterbox.
No matter which way you mount your iPhone to a tripod, you should check out Joby's free app GorillaCam. It's a nice suite of features that let you be more precise with your smart phone photography.
Here's a quick time-lapse sequence I made to test our the tripod mount while working on my bike:
Step 1: Remove Belt Clip
The first step is to remove the OtterBox belt clip from the holster. I thought about incorporating this into the tripod mount to have the ability to swivel the iPhone but in the end decided just to go without it.
First, to remove the spring-loaded plastic piece, you just need to pull out the bent metal spring with a pair of pliers. Then this piece will come right off.
The plastic piece attached through the swivel to the holster was a little harder. You might be able to pinch together the inner tabs and release it, but after trying for a while, I just took a dremel tool to the tabs.
After that everything should come apart into 4 individual pieces of plastic and the metal spring.
Step 2: Modify Holster Shape
To control the iPhone while it is attached to the tripod, the screen must face out from the holster. This means we must cut the holster to clear a view for the camera. A small hole around the lens would work, but I wanted to trim off more to make a smaller, better-looking package. I followed seams and casting lines on the holster, using a band saw to cut and some sandpaper to clean up the edges.
Step 3: Build Tripod Attachment Point
This is where most of the work comes in. I chose to make two mounts: One is in the middle of the holster, so the iPhone is balanced and will be steady on pocket tripods. The other is directly underneath the camera, so that when panning video or taking panoramas the axis of rotation runs through the lens. A standard tripod has a 1/4" 20 tpi bolt that attaches to the camera bottom. We implant a matching nut into a small piece of wood, eventually attaching the wood to the holster with screws. Here's how:
Measure the distance between the center of the iPhone and the lens, and cut a small piece of wood to accomodate two holes this distance apart. For my iPhone 4, I placed the mounts roughly 1 7/8" apart, and my wood piece is roughly 2 5/8" long, 3/4" wide, and 3/8" tall.
For each mounting point, we drill with a Forsner bit a hole slightly larger than the nut into the wood, stopping just a hair before going all the way through. In the center of that hole, we drill another hole all the way through, slightly larger than the bolt.
Drop the nut into the hole, and thread a spare 1/4" bolt from the bottom so the end of the bolt is flush with the top of the nut. Pull the nut tight against the bottom of the hole, and pour epoxy on top. When the epoxy has dried, remove the bolt and sand any protruding epoxy flush with the wood.
Step 4: Put It All Together
The last step is to attach the modified hip holster to the piece of wood. After lining up the mounting points with the lens and center of the iPhone, I used three small wood screws, drilling pilot holes and countersinking them in the holster.
All together, a nice little package.