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.