I would not thing there would be a problem if you can keep the intensity up. When I was in the police academy part of our morning PT involved boxing for 3 minute rounds (paired off, one person throwing pre-determined combos, the second person wore the mits) the switching and then wearing the mits for the other person for three minutes and it was quite the workout.
shadow boxing is great for hand speed and its good for yer cardio to It gets my heart rate up fast and gets me breathing hard as long as Im woring on my speed beter than runing does but when it comes to conditioning you need a little bit of everything
I've been doing it for about a month on off days since there's very little else to do in a college dorm room. It works pretty well, but as others mentioned it's no replacement for running.
I recently read a magazine article about it, so people seem to be jumping on this idea.