Good, someone actually did the math, can't let the thread die here. I was thinking that it would be double the current 800 meter record or 3:22 (3:23-4). That would necessitate someone who could easily break both records yet be specialized for the mile though, not likely.
It's our past that is to blame for our limits. Our legs are adapted to run from those of our ancestors who didn't really use them for that. Look at an ostrich for comparison, way better design and way better results.
Beyond that is fitness. VO2 max, LT threshold, ect. I don't see this improving too significantly past current leaders unless someone manages to incorporate cross training in a way that boosts these numbers to levels seen outside of running in athletes completely geared toward running. Even modest improvements here would synerize with improved mechanical efficiency though. A runner who can come though 1600 in 3:30 using only the effort per minute as another taking 4:00 also has the advantage of taking less total time and thus can go out more aggressively.