Some of you people are making predictions based on prior experience, and that doesn't hold much weight here. Phelps is obviously in better physical and anaerobic shape then most anybody on the planet, but have any of you seen him walk on land? Or seen clips of him jogging?
To put it bluntly, he's horrendously bad at being land animal. His torso is massive, his legs are short, and his coordination is impeccably bad. Obviously he could take time to train his body to run more efficiently, but stop comparing Alan Webb, or your high school swim buddy, to Phelps. They all had different body types...Phelps' body is perfectly suited for water, that's one of the major reasons he is so good and efficient at swimming.
So if the question is, right NOW, what can Phelps run for the mile, I would put aside any notions of VO2 max or anaerobic ability, and just assume it's going to be slower than most of us. Give him some weeks of form training, and maybe he could run a decent time, by that I mean somewhere in the low 5s.