A group of pro-democracy student leaders asked in a joint statement that China allow free travel. They and many others have had their Chinese passports revoked or renewal denied, effectively banning them from returning to China. Their joint statement says that the Olympic Charter prohibits any country from denying anyone the right to attend the Olympic Game on the basis of political or religious affiliation, and that China engages in political persecution via its passport administration, thus violating the Olympic Charter.
Source: Asian Times, March 6, 2008 reprinted by Boxun http://news.boxun.com/news/gb/intl/2008/03/200803070458.shtml