Yes Yahoo has handed over details of Chinese dissidents and journalists to the Chinese authorities.
As an example Shi Tao was a journalist who forward on to human rights groups an email from the Chinese Government which told journalists not to cover any protests during an anniversary of the Tiannmen Square massacre.
The Chinese Government got their hands on the email and asked Yahoo who sent it.
Yahoo coughed up the journalists name.
The journalist got 10 years.
A member of the US Government put it quite nicely when he said "technologically and financially you are giants, morally you are pygmies"