That will never happen.
The Chinese are not out of their minds, and a first strike by the US would erase more than 75% of China's very tightly packed population along with 100% of its industrial base and 100% of its agricultural land.
The problem with Iran is that Ahmadinejad, unlike Jintao, is actually insane. Completely bonkers.
Go back and study the history of First Sino-Japanese War and you will understand why I believe this.
One Chinese general made a comment about losing a half billion as acceptable. However historically the communists, even with the Soviets, were for the most part not that insane since they were aware of their own mortality (although there was one time it got close in Eastern Europe, until Colonel Kuklinski leaked the Warsaw Pact warplan to the CIA). With Nutjob however, he views being annihilated as a quick trip to Paradise. Except the final destination is going to be a little warm. Then again maybe he likes the smell of sulfur.