I would imagine one of the weaknesses of this kind of schedule is the variations that occur when you have an evening function that you just need to go to, ie., birthday party of your best friend. And what happens when you have a few evening functions you need to go to in a week? I would imagine that would mess up one's rhythm. Sure, you could try to get back to your schedule with diligence and effort, but I can't imagine it's that simple. Over time, that might become draining. But it seems like you get so much stuff done from midnight to 8am, maybe it's worth skipping the evening functions altogether or if you need to go just re-adjusting to your schedule (even being a bit groggy) because of the huge productivity gains you're seeing.
One of the interesting things from the blog post was how it's easier to concentrate when the Internet slows down. I find that 9am to 5pm on weekdays there's just so much content being published everywhere, and so much of that content is interesting, that I find myself easily distracted during the day. I've had to hack my own system of focused goals, pomodoro work sessions, and other homemade hacks to get the productivity I need during the day. So, I can totally see how someone can get tons of stuff done during the middle of the night while everyone else is sleeping.
Btw, Kevin - how long have you been doing this schedule?