Nadal won Monte Carlo, Rome, and Madrid -- all three 1000-level clay court tournaments that lead up to Roland Garros. I can't come up with a perfect mainstream-sport comparison, but even taking into account the Soderling loss last year, if he's healthy, I think Nadal losing at the French this year would be like Tyson losing to Buster Douglas. He is a million times better on clay than everyone but Federer, and he seems to own Federer anyway.
There are some other very good clay-courters out there, but they all seem to fold the moment Nadal is across the net from them. And to be fair, Nadal is amazing on the surface.