I often walk the same MUP (Silver Comet Trail) as I ride and I can say 100% that it scares the pooo out of me to have a bike go flying by at 20mph without ever announcing..

When I am on my Trek 7.2 I ding my superbell as far away from peds as I could reasonably think they could hear it, gives them plenty of notice that someone is coming.. On my roadbike I usually give an "ON THE LEFT" in my best command voice as far away as I would expect to be heard.. Not much you can do about the Ipods...

It is really one of my pet peeves when someone is coming up around me and doesn't bother to announce, makes me want to swerve into them on purpose..