What I like about the mens' field is that under the right circumstances there are a number of players who are capable of beating Roger. These men don't seem to be intimidated or defeatists when they step on the court against him. I feel that Henman and Hewitt have the best game and mental/physical fitness to do that right now.
Roddick has the desire, but, he needs to be able to sustain controlled agression for too long a period of time to snatch a match away from Federer.
Nalbandian and Ferrero aren't healthy/fit.
Marat has the talent, but, he needs more than mental strength to defeat Federer. He has to maintain a constant off court regimen and learn how to employ an on court strategy for an entire match in order to even challenge Roger. I hope he stays with Peter, listens to Peter, and executes.
I don't know if anyone can defeat Roger if he makes it to the final of a major because he'll try to peak for that, but, there will certainly be challengers at smaller tournaments and in early rounds of majors.