This is an interesting thing to consider in my opinion since it may be possible that Nadal will not play much in 2012 anymore.
The current race ranking: http://live-tennis.eu/race
Basically, Murray is 2100 points behind Nadal in the race whereas Ferrer is 2800. Del Potro is about 3500 back. If Nadal does not play during the remaining year, I think that it's very likely that Murray passes him. Andy is bound to collect 2100 points in the remaining tournaments and Ferrer might be close as well, especially if he gets nice draws (being the nr.4 seed) and gets to the semis of big tournaments often. Del Potro is an outsider here IMO. If Nadal is fit to play after the USO, then I don't think there's a chance he falls outside the top 4.
If Nadal were to finish the season as nr. 5, it would mean a very interesting AO draw I think...
What do you guys think: What will Nadal's year end ranking be in 2012?