I've always looked at it this way, which may or may not be true: If you're a good player you should play from the very start. You get more time to capitalize on your skill and playing deep is to your advantage. If you're a bad player you should enter as late as possible. The less time that's left of the tournament the more luck comes into play, which is what you want. Also, the shorter the stacks the better.
However, a major flaw in this reasoning is that big time pros regularly buy in late, so I just don't know...
Very interesting to see the ICM numbers, though!