Like some of you, I realized early on that the iPhone is a wonderful device. As an occasional cycling enthusiast I like to keep track of my rides. I've used several bike computers that have worked well, but really fell short in some areas. I downloaded the Cyclemeter App and decided right then and there that I was going to toss my bike computers and use this amazing cycling app. There was a problem, however. I didn't like any of the iPhone bike mounts I'd seen on the market. I didn't trust them because I cannot afford to replace my phone of it departs the bike mid-ride. It took some thinking before the solution (which was sitting unused in a drawer) came to me. It's as simple as it gets and if you already own an Otterbox Defender case for your iPhone 4, you've got 90% of what you need to make this work.
You'll need: the Otterbox Defender case and belt clip (included with case), several medium length zip ties, pliers for tightening zip ties, cutters for trimming them down, and as an option you can use some craft foam between the clip and the stem of your bike.
Cut a section of craft foam the same size as the belt clip. Being a student teacher, I have lots of this on hand. I like the sticky back variety. This is simply something to give the clip something to cozy up to other than the metal of the stem. I think it helps prevent the whole thing from rotating down to the sides.
Place the foam on the upper side of the stem.
You can put the zip ties on now, or after you place the clip on the stem. It's up to you. If you maintain a bike, you can figure this out.
Place the belt clip on the stem and put the zip ties over the clip till it looks something like....
Tighten the zip ties while you make sure the holder is centered on the stem.
It should look like this. The belt clip/case on the iPhone 4 Defender allows the phone to clip in either face down (covered) or face up (uncovered). I tried my wife's iPhone 3gs with an Otterbox Defender and it did not work. Here are some images of what it looks like in the riding position. The clip rotates and will click into any position (landscape, portrait or anything in between).