In Arlington, Virginia, a man yelled "to your left," causing an 80-year-old woman, Ita Lapina, to step to her left. She was struck and killed.
Around here, they shout "on your left," not "to your left," but I'm not sure that clearly avoids the ambiguity. Basically, shouting at a pedestrian from behind is startling.
Bikers, think of Ita Lapina, and get a bell. People instantly understand what it means, and they don't have the confusing experience of suddenly finding someone yelling at them. Quite aside from the safety issue, it's irritating to be yelled at. I bike on bike trails often, and I know exactly what's happening when I hear "on your left," but on a sub-rational level it feels rude.
Also, when you're passing a pedestrian — especially an older person or a child — go slowly and give them a really wide berth.
ADDED: In the comments at the link, someone says: "I'd have no problem with banning full suspension bikes from multi use paths. Not because they're fast, but because cheap ones have handling issues and are too often ridden by poor cyclists." I don't know anything about whether that's true, but I see in the article that the bike was a Next Powerclimber. That bike costs $88 at WalMart. That's amazing. They'll even ship it free at that price.