Hi hows it going! i've been looking at this problem for some time as a few of us are riding to oz via mongolia, china next year.
There seems to be a lot of people saying i've heard this and that and there not letting anybody through, i think what you need to look at is who has actualy tried to do it and physicaly been there, because if they havent they don't realy know what there talking about,they just heard stuff. As far as i can see only a small handfull of people have been refused many years ago but recently i've heard from poeple that have crossed the boarder this year, no one this year has been refused as far as i know, but i havent personaly been there so take that as you will.
I have written a letter to the chinese embassy in london this week to find out what the score realy is and i'll keep you posted. Other wise do what everyone else seems to do, chuck the bike in the back of a truck and meet it at the first town over the boarder, the siberian express runs through the mongolian/china boarder so you could even jump on that.
hope that helps you out anyway