I recently bought a RAM iPhone mount for my bicycle, and it has made cycling a much more enjoyable experience. The mount is simple and secure using two strong zip ties and a rubber wrap around the bars. I mounted the iPhone on the stem in the middle of my handlebars. In conjunction with this I purchased the Cyclemeter ($5) iPhone app as it suited my needs the best. In fact, my old bicycle computer is quickly becoming obsolete.
All this has changed my cycling habits. Now I get on and press start; it keeps track of where I went, how fast I went and how long it took with maps and graphs. It shows what days I biked on a calendar. When I take the same route, it compares it to previous times I have followed that route. Everything is stored on the iPhone, but the information is easily uploaded and shared in various ways and formats.
[Note: This mount is for iPhone 3G and 3Gs. Ram produces mounts specifically for iPhone 4 and other GPS enabled smart phones.--OH]