I've just posted on the french forum and realised it might interest others.
While in Laos 2 weeks ago, we met a couple who had been refused entry in China in Boten. The reason invoked was that it was not permitted to cross the border on a bike. A few days later, we met another cyclist who had had the same info by the chinese embassy of Vientiane itself. They also said that putting the bicycle on a bus might be ok.
Does any body have more information on the subject ?