I think people got carried away by him beating a low confidence Nadal in 3 sets Masters clay finals. Those wins said more about Nadal's state of mind and state of play, than Djoker's brilliance on the surface. Nadal has not been at his best on clay this year at all, but may just be peaking for the French Open final.
Djokovic is a very good clay court player, but winning Seven Best Of 5 matches at Roland Garros may be beyond him. This year was likely his best ever shot at the title, but he couldn't get past an aging Roger Federer in the semis. Even if he got to Nadal, the occassion and Best Of 5 format would probably have gotten to him.
Next year will be a different kettle of fish. The "streak" is gone, and probably so will any sense of invincibility. The likes of Murray now present a realistic challenge on clay, and could stop him at Roland Garros. Soderling and Federer could probably stop him as well. Nadal now knows that he doesn't even have to play that well in a clay season to make the French final, and probably win it. That's a magnificent ace in the hole....like Serena Williams playing like crap in tune-ups, but playing her way into form to win slams.
If Djokovic couldn't do it this year, it's not going to happen. He's lost the element of surprise. Others are improving, and I think Nadal will be motivated to try and win more RG titles.