There was extensive talks about it over at tennis.com and it's nearly impossible to calculate, depends (should Roger not defend his points) - what round he would lose, and whether Rafa wins them all instead - and if not - which round he'll lose in.
But the talks was that there might be a real battle for number one in Shanghai or Paris.
True. We have to wait to see what he does during the summer hard season to see what he needs during the USO to stay #1.
Just to give some ideas though, here are some possible scenarios:
-Nadal wins Stuttgart
-Federer and Nadal both lose in the SF's in Canada
-Federer wins Cincy and Nadal is runner-up
For Nadal to get to #1, Fed would have to lose in the first round of the USO and Nadal would have to win it. If Federer wins one summer TMS event and is runner-up in another, he will be number 1 through the USO no matter what Nadal does.
--Nadal wins Stuttgart
--Federer loses in QF of Canada and Nadal wins
--Federer is runner-up in Cincy and Nadal wins
--If Federer loses in the first, second or third round of the USO, then Nadal becomes #1 by reaching the 4th round.