I had such a tour with a travel agency in Xi'an three years ago. But I am not sure if they have the tour or not now.I just got an e-mailfrom the guide,you can ask him at:ymasx@yahoo.com
Good luck!
Simon

Hi I have worked for Mongol Global Tour Company for the last three years. They have been in business for almost 10 years. They are affiliated with BBB, IATA, ASTA & PATA. If you are interested in speaking with people who have traveled with them in the past just ask for a reference list. This company can provide highly personalized service. They are also recommended by Frommers Guide to China, DK Eyewitness Guide to China, and Bradts Guide to Mongolia. I think it is the only thing missing out of the Fodor's book

I am sure blancalita has been to China/Mongolia and back several times now. But thought I'd respond to this original ancient post.

I traveled with them in 2005 to Mongolia and spent an overnight on the Great Wall. It was a great trip that included private and group travel. Transportation in Mongolia throws some curves. We had to adjust to them, but it went well.

In 2006 they arranged for me to volunteer at the Wolong Nature Center for Pandas in China. This was all private. Another excellent adventure.

I posted both trip reports here. Feel free to email me if you can't find them.

I will use them again some day.