Just to add my two kroner - I wear a not-an-ipod bike commuting, and I can easily hear a bicycle bell at any distance within which I have to do something about it. I can also hear traffic, horns and sirens ditto. Sure there are many clueless gits out there, but wearing an ipod doesn't necessarily mean you have it on full blast or are not paying attention to your surroundings.
I am not starting a discussion on ipods on bikes, just don't not ring your bell behind an ipod-wearing runner because you assume he or she can't hear you.
I use a bell and find it very handy for warning people well in advance that I am approaching "their space".
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett