I've tried both methods and a bell seems to cause the least amount of panic or confusion. Yelling "on your left" will sometimes cause people to scoot to the left (¿no comprende?), while the bell just plain notifies them that there's something coming up from behind. It puts all of the responsibility on them (if they can hear it). If they don't look back, all I can do is assume they're aware I'm back there, deaf or plugged into their iPod.

One jogger took me by surprise about a month ago and actually thanked me for belling him as I rode by.