i'm not trying to say that nadal is the greatest. i am just saying that it is too early to call federer the greatest at this point. he very well could be. but i really believe that he got to a very high level, with not much competition to stand up to him. until nadal.
I can't predict the future so i don't know what will happen.
Both Nadal and Federer could go on winning for many years (more likely Nadal then Federer) or one of both could decide to retire tomorrow.
if that was to happen (retire tomorrow), it would be certainly arguable that Federer -even without any additional win- is the best ever because, among the only 6 players to have won all 4 Slams, he has won the most, or because of the only two to win 14 Slams, he is is the only one winning all 4 (as Sampras never won Paris).
If you look at total single titles won, he is currently 8th in the all-time standings with 59 wins. Connors and Lendl are probably unreacheable (by Federer) at 109 and 94, by he's likely to pass sampras (fourth) at 64 and maybe even McEnroe who is third at 77. So right now he is not the one with the most titles, nor it is likely that he ever will, but i think we both agree that quality of wins is as or more important that sheer numbers.
As far as Nadal, If he can keep healthy, he might well become the next "greatest ever"; as of now, he is not. But if he does keep winning, we won't have to wait for the end of his career to hail him, just until the point when his achievements will have topped those of federer, sampras and all the other 'greats' of the game. And if -when he does- there is another up-and-coming young gun who will be plying better than him at the time, that should have no effect on the evaluation.
As a matter of fact, Nadal should already be numbered among the very elite, certainly "one of the" best ever. Just not "the", yet.