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
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
Step 3: Build Tripod Attachment Point
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
All together, a nice little package.