Nadal had to retire due to injury. Unfortunate, but I think Murray was going to beat him this time anyway.
Serena seems unstoppable to me. It's always a tossup when the sisters play, but I think Serena's got the edge right now.
Davydenko is also playing very sharp tennis. He's so small and light that his movements (direction changes) are deceptive. And he's such a low-key personality that it would be amazing to see him win.
The smart money has to be on Federer, he's still the #1 seed, but I like Murray a lot for this year.
Poor Nadal is facing some nasty physical challenges. I'm afraid his 2010 season may be shot, but even worse, his career may be in jeopardy if he can't get ahead of his injuries. His game style is so physical that I don't think he can compete without 100% health.