<Update: December 19th 2013>
Four years after writing this post, it's still popular, and was recently featured on MakeUseOf.
I'm pleased to say that in the past four years I've upgraded my phone twice (Most recently to a Nexus 5), and am still using the original mounting bracket. It's been holding up and serving me quite well!
Riding my bicycle for over an hour a day as I make my way to work and back and around the city, I've often felt the need for a GPS. At other times, I would want a speedometer, so I'd have some idea at what speed I'm taking that slope.
In this post, I'll show you how to safely attach your cell phone to your handlebar for less than $5.
Ever since I got my Nokia 5800, it dawned on me that this device would be a great addition to my bike. Like many other smartphones, it has a built in GPS and a huge screen. If only there were some way of easily and safely attaching it to the bike...
In my search for a solution to this need, I found this cheap universal cell-phone mount and went and ordered it. Unfortunately, this mount is really bulky and as a result shakes wildly with every bump in the road. I did not feel safe having my phone in it. In addition, its big thumb screw makes it an easy target for thieves. This was clearly not the solution for my needs. I envisioned the phone flush with the handlebar. This was far from it.
I ended up making my own phone-mount, and it was as easy and cheap as can be. I took a detachable flashlight mount (similar, though not identical to this one), and glued it onto a silicone case I bought for my phone.
Remove the clamp that's meant to hold the flashlight. I used a pair of scissors to cut off most of the unneeded plastic, and then sanded the rest down with a leatherman. If you buy this one ($2.98, free shipping), it looks like the clamp unscrews, and you can use it for some other project (attach it to your helmet, maybe?)
I lightly sanded down the area on my silicone case where I was planning to glue the mount, in order to make sure the surface was free from chemicals & coatings, and to make it a little rougher, so the glue sticks better. Apply a thin, even layer of superglue, and firmly press the mount into place. Let it dry for about 20 minutes, and slide your phone into your new bike accessory.
You can find good silicone cases for almost every imaginable phone here; many for under $2.
So what does one do with a large-screen dashboard on one's bike? I've used MGMaps to get directions and navigate unfamiliar neighborhoods; Nokia Sports Tracker as a big-lettered speedometer and odometer (and as a bonus, it records your route so you can later analyze and share it); and Waze for voice turn-by-turn navigation. In addition, having your phone on your handlebar rather than your pocket allows you to see who's calling you. Video calls also work with the phone on the handlebar, pointing up at you.
I've been stopped by fellow cyclists on several occasions and asked about the GPS on my bike. I hope that this guide will get some of you guys out there to try this for yourselves. Just the bewildered looks you'll get from drivers around you will be worth it.