Then shouldn't the effective prohibition of bitcoin in China exert some kind of upward price pressure as well?
If we're talking about this as an isolated factor, no but maybe. In the short term, you'd expect a large number of legitimate people to dump their holdings and tank the price. In the long run, the price may be driven up by demand, but only if it's substantially harder to get them into the country than "install Bitcoin client". If it's just as easy to use and trade them in the future as it is right now, they don't have the low supply & high demand situation necessary to artificially inflate prices.
