Dirty or not, how the hell could the fluid have any effect on the ABS sensor? The sensor is an electrical doodad that goes into the hub and is in no way related to the fluid. Maybe you meant the ABS control/pump unit that controls the pulsing? THe fluid does go through that, but it still seems far fetched that fluid backing up through it from the calipers would mess it up.

I didn't open my bleeders the one time I did my brakes, but I probably bled them right after anyway. I've replaced the pads once and I think I've replaced and bled the fluid about 10 times!