Nadal will beat Fed in the Aussie.  There are three reasons why.

First, Nadal's basic groundstroke drives Fed nuts.   The Fed hates high backhands and forehands.  They require him to work more, and he hates more balls out of his zone.  Thus forcing him to get into long rallies.  Normally, Fed would just come to net, but Nadal is a demon on the court, his coverage is absurd, and his passing shots are way too good.  Hey i just found out, the ITF has a new ball--that is slower, and courts have been made slower.  That's bad news for Roger.

Second, Roger's serve is neutralized by Nadal's game.  Nadal can return most of Roger's serves, but the sheer length of the rallies consumes all that energy that Fed would normally use in his serve.  If you watch Roger enough, the matches he dominates are the ones where the points are three shots or less.  Few people can stay in the rally that long with him, but Rafa can.  

Finally, the Aussie is a marathon now.  It's not as grueling as the French, but with the high temperatures--you ever play tennis in a bowl that blocks the wind.  The Aussie is tennis in a bowl under the sun.  IF it rains, and they close the roof, turn on the air conditioning, Fed wins, but if the match goes 4 sets, he loses.  I really think Nadal will wear him out within the first set with his high loops, great gets, and improved serve.  Throw in 30 degree weather and humidity, the Fed will wilt.

Tennis now a game all about defense--Verdasco almost hit 100 winners in his match with Nadal, but Rafa had 5 unforced errors in set one, 5 in set two, 4 in set three... I mean how do you do that?  when the sets are 6-7, 6-4, 7-6 those are max number games and you're hitting 5 unforced errors?  Unless the Fed is on fire, Nadal takes this.