Not sure why this question comes up so much here, but I do feel obligated to answer, being one of the 'slow' guys. I thought I would have had trouble breaking :55, but that was based on times I ran in workouts (struggling hard to break :27 in practice), in a 'race' situation who knows - maybe I could have gone :53 or :54, but it would have had to be with a running start.
15/mile was my best event by far - no raw speed and only decent endurance (even then only up to 3k or so) and I was pretty good at running pace. I imagine if someone was great or perfect at the mile distance (solely as it pertains to their overall ability), they could get away with something in the :56 - :57 range. That person likely doesn't exist, but I do think in theory it could happen.
Those of you saying sub :50 (and to some extent those of you saying :51 or :52) are way off.