check out gt-rider.com forums, theres a brilliant story about going from Laos to southern China and even Tibet about a year ago. But this guy also went with a chinese-plated 200cc bike, I would imagine its a different story at the border if you got another plates. He even ended up selling that bike and buying another one, a new 200cc offroad, but then he had an accident and needed to return to Thailand for treatment. Still one of the greatest travel reports Ive read!!
From whats been written in here, doing China the "official way" seems to be possible, too, but not without plenty of time to arrange everything (a mandatory guide, for example) and a very, very large budget.