Um, i think the older Federer gets the harder it'll be for him to beat Nadal, other than physical reasons.
Main reason:
To neutralize Nadal's forehand (even on the hard courts) you're going to have to time the ball exceptionally well, or risk dropping the ball short or long. Both would probably give Nadal the point or at least give him the advantage in the rally. But as Fed gets older and older, his timing to take the ball early would deprove accordingly, making it more difficult for him to come up with winners on a consistent basis. Hence, i see him continuing the trend of giving Nadal a really good fight, but coming up short most of the time on important points like break points, and maybe even match point.
But having said that, Fed on a good day where he is on the zone and serving well would definitely give Nadal a good fight, and i'd take him over Nadal on the faster courts.