The obvious thing to do with a portable device which knows where you are and can display maps as your location changes is to use it for navigation and tracking. This obviously requires that you put the thing where you can see it as you go along.
There are, so far as I could find, two bike mounts for the iPhone 3G available in the UK. I bought one of each, so that I could be sure of owning the best one. Happily it is easy to tell which is best.
This is undoubtedly the worst one. There is no way to fit it into the phone-holding component without the frame touching either the silence switch or the suspend button; and whatever you do, either the microphone or the speaker will be obscured. Another flaw is that it’s designed for the way handlebars were built ten or fifteen years ago: modern bars have a larger diameter where they meet the stem and blend between that and the narrower diameter where your hands go; this mount will only fit on relatively narrow bars of constant diameter. The mount can only go on one way around, so your phone can only ever be in portrait mode. Finally, the hold on the phone is not very firm at all, so if you persevere despite the aforementioned problems you’ll probably be unable to see what your phone is doing while it vibrates itself to death as you ride along.
Happily the other option is rather better. The grip on the phone is much better, although it’s worth putting a bit of padding in the back to make it even firmer. You can easily switch between portrait or landscape orientation whenever you like. Because it is tied on with cable ties it will fit on bars of any size; I have mine fitted to the stem, as there isn’t a lot of space left on my bars. They supply a wedge back for the part which attaches to the bike to solve the tapered bars problem; although I am using the wedge back for a different reason: to angle the phone so it’s easier to read when I’m out of the drops.
So there you go, quite an easy choice. You will also need some software to go with it. I am using Trails on the iPhone and TrailRunner on my MacBook Pro. Unfortunately TrailRunner is more oriented towards running than cycling, although I think it is worth the asking price as-is (it’s free but asks for a paypal donation). If the author is reading this and wants to let me know how much of a bribe they want to make it more cycling-oriented, they should feel free.