Get ready for some numbers...
Roger Federer just lost again to Albert Montanes in the semifinals of the Estoril Open, meaning he picks up 0 points from this tournament (90 points for semifinals, but it only replaces a 90 point tournament he already had).
If Nadal follows expectations and wins both Madrid and RG, he will pick up AT LEAST 3420 ranking points on Federer (Rafa would gain 2220 points, and Federer would drop at least 1200 points since he won those tournaments last year). That would put Nadal around only 600 behind Federer. Even less if Federer doesn't get to the finals. All Nadal would have to do to gain #1 after that is make it to the semifinals of Wimbledon. Add to that a poor summer hard court season last year and Nadal could really start to get a strangle on the #1 spot.
Granted, Nadal still has to win Madrid and RG first...