I’ll be using my iPhone as a cycle computer, which means GPS. I use the camera, likely shoot some video and who knows what else. It is so capable. My reading [warnings] tell me of its short battery life. Maybe so, but not for me. As a user of an Edge 305 [which has failed to get me through a century or two], I now use an external power source. For the Edge, I’ve used a Li-Ion USB external battery [I have 3 of these] and a simple 2AA battery USB external battery. They both work and have worked on two double-centuries for me.
Testing, my iPhone, I’m on my 3rd day of running it on my external Li-Ion battery—not a lot of drain, in that I using its ipod function and occasionally use the browser or other apps. My point here is that the Li-Ion connected to the iPhone will probably get me through any cycling event I’ll participate in. In my testing, the 3GS has yet to go to its internal battery.
YMMV; but, from my perspective, if you are doing "endurance/extended" events use external power, rather that expect the iPhone’s internal battery to get you to the end.
My $0.02, FWIW.
Works for me!