Most of the times I've encountered mountain bikers with their dogs, the dogs were well-behaved and it was fine. A couple of times though, the dog was poorly trained and had no qualms about darting in front of bikes or chasing. I haven't been bitten or knocked over yet, knock on wood. I guess the issue is that everyone thinks their dog is well-behaved and would *never* do anything bad, but clearly, since it happens, that's not the case. It's the exception rather than the rule though, so I can't complain too much.

For the record, I care about dog poop a lot less than the huge piles of horse sh*t that take up the entire trail.