Canon Europe has produced an online depth-of-field calculator that allows you to input your camera, aperture, focal length and focus distance and get an almost instant calculation for your required near and far limits of focus, total depth-of-field and hyperfocal distance.
They also offer a very good article about DOF. An excerpt on focal length:
"You will usually choose the focal length to suit the subject rather than to suit the depth-of-field. However, the accepted rule is that you get more depth-of-field with wideangle lenses than with telephoto lenses. In fact, this rule is misleading. What actually happens is that a wideangle lens magnifies the subject less than a telephoto lens, which means that more of the image appears sharper."
Update: Yes, the Canon DOF calculator works on an iPhone.