I hope we're talking road bikes here? I commute on a mtb, at least at this time of year, and I'm guilty of never ever checking my tire pressure... (unless picking my bike up and bouncing it a couple of times counts )

But then - if I flat I have all of 500 yards or so to the closest bus or metro stop, and I have tubes and stuff at work. On longer rides or rides out of town I'll carry it all, of course.