Agree w/Mark and John.
This is the time of year when they are fattening up. The place I camped last weekend actually had a warning out to everyone there. Saw some matted grass with a 1/2 eaten rainbow trout in it that provided enough proof to be cautious.
I have heard that clapping your hands over your head and making loud noises usually does the trick.
I would stay away from any mothers with cubs around though. From what I understand most incidents happen when a curious cub stumbles upon you and then big mom shows up.