Over on Superbikeplanet.com, there's a great picture of Rossi's ruined rear tire, which caused him to slow and, Yamaha claims, caused his bike to overheat and blow coolant all over the track. I can't post it here because of copyright reasons, but here's a link to it:
If you look at the left-hand side of the tire, you can see a large strip where the rubber has let go. The interesting part about it is that the line of damage seems to be more or less along the line where the two different compounds in the dual-compound tires are joined. After Rossi's front tire delaminated (or to put it in layperson's terms, blew a chunk of rubber from one of the layers) in Shanghai, there was some speculation that the Michelins are not well suited to the Yamaha's power and handling characteristics this year, especially as the Honda riders have not had anywhere near as many tire problems so far this season.
Now, the following is all idle speculation, but I hope you'll bear with me, as it could be interesting speculation: I'm sure you will all remember Max Biaggi's performance on the main factory Repsol Honda last year. He spent almost the entire season complaining of chatter, and his complaining probably cost him not just his ride at Repsol, but any other possible ride on a Honda for 2006. Put in a rather oversimplified way, chatter is caused by tires generating more grip than the chassis can handle comfortably. During the 2005 season, virtually no complaints were heard from Yamaha riders about chatter. It seems logical that Michelin would choose to focus the larger portion of their development efforts over the winter on building a tire which would suit the Honda, especially when you consider there are 6 Honda RC211Vs (plus the Team KR KR211V bike) running on Michelins in the MotoGP class. It is quite possible that all this development work turned out much better tires, but tires which suited the Honda better than the Yamaha.
You may argue that Michelin loses out by not providing their current world champion, Valentino Rossi, with the best possible tires. This overlooks the fact that all of the candidates to take Rossi's title from him are on Michelin-shod Hondas. Either way, Michelin will be able to claim its tires are used by the MotoGP World Champion.
Another noteworthy item is that all of the photos and publicity released about Michelin's test rider Jurgen van den Goorbergh (though, frankly, this has been very, very little) have shown him riding a Yamaha M1. The tests are focused entirely on developing the tires, rather than the bikes, as van den Goorbergh is under contract to Michelin, but it's certainly interesting that so far, as far as we know, the tires have been tested on the Yamaha, rather than the Honda. Nicky Hayden's 51 point lead in the championship race is, of course, eloquent proof that the Michelins work just fine on the Honda.