I don't post a lot here but I am a good reader of these forums. So this is my first great thread
The way I see it, Del Potro will be no.1 next year.
He's already a major winner, he's progressed more than Murray, Djokovic or Nadal in the last year, and he beat Federer and Nadal in their last meetings showing nice mental strength. He represents the new era of power tennis. He's been struggling with injuries since January so he was in poor form in the AO and missed IW and Miami. But I think a healthy Del Potro will be scary, especially looking at how the other top players are performing:
Djokovic forgot to play aggressively and has become a grinder + mentally weak.
Murray didn't learn to play aggressively yet, he is the same player than 2 years ago, hasn't changed anything. He's being outpowered and losing too many matches.
Nadal struggling to reach his 2008-early 2009 game. Not as confident as before, making too many mistakes, his injuries are always a concern.
Federer, a great player in Slams but too inconsistent the rest of the year + he will be 30 in 2011.
So I see Del Potro being no. 1 in 2011 barring injury