The huge difference between China and the Soviet Union is that China has already embraced Western culture, the illusion of democracy, capitalism, etc. They love our technology, as opposed to the Soviet Union that kept western advances away from their people (due in part to the lack of wealth to purchase it, etc.)
The problem with China, as stated, is an underclass of farmers, etc living in a near 3rd-world environment...but to suggest that all the farmer's will up and rebel someday is not accurate, either. One, they can't organize for any number of geographical, educational and cultural reasons, including language. One of China's biggest problems now is keeping people employed and happy...but with an economy growing at about a 7% clip right now (down from about 9%), and industry coming in, and the building trades *booming*...they are making progress towards their goals of stability.
sometimes that means taking entire villages and sticking them in the new city they just built...and the people do it...
Anyway - the point is - China is a vastly different situation than the Soviet Union (bread lines, etc.) and isn't in the situation of the Soviet Union (bankrupt due to the Cold War/Military spending) and therefore would not collapse as the S.U. did.