It happens, how often? Wouldn't want to find out.

"Americans doing business in China should be aware that if they become involved in a business and/or civil dispute, the Chinese government may prohibit them from leaving China until the matter is resolved. There are many cases of American citizens being prevented from leaving China for months and even years while their civil cases are resolved. Civil cases may sometimes be regarded as criminal cases and the defendant may be placed in custody."
http://shanghai.usembassy-china.org...._disputes.html

China LawBlog wrote a three part series on this in 2010.

http://www.chinalawblog.com/2010/05/...ped_in_ch.html

http://www.chinalawblog.com/2011/01/...re_you_go.html

http://www.chinalawblog.com/2012/06/...re-you-go.html