What nybiker said. I call these people yo yos, and they drive me nuts. Just make sure that you announce yourself as you pass with an "on your left.".

As for riding someone's wheel, that's harder to say as it really depends on the riders in question. I don't generally mind it, but I give a little extra distance when riding a stranger's wheel. If someone's riding your wheel, make sure to signal or announce any raod hazards, stops, etc.