As the Chinese government does not comment on their practice of specific censorship, only user reports are available, and they appear to be spotty and varied.
According to the Shanghaiist, any search, political or inane, on both Google.cn and Google.com.hk were met with a "connection reset" and even a search for "shanghai" was met with "the white screen of death".
The local Shanghai blog also quoted some as saying that a possible reason for the block might be the inclusion of the letters "RFA" in the Google URL parameters. RFA can stand for "Radio Free Asia", a site that is also blocked in China.
Update: According to a statement from Google earlier today, the inclusion of those letters was indeed the case for the blockage.
"In the last 24 hours "gs_rfai" started appearing in the URLs of Google searches globally as part of a search parameter, a string of characters that sends information about the query to Google so we can return the best result. Because this parameter contained the letters rfa the great firewall was associating these searches with Radio Free Asia, a service that has been inaccessible in China for a long time - hence the blockage. We are currently looking at how to resolve this issue."
Update: [March 31] Google later reversed its statement, saying that the "rfa" part of the URL had been added a week earlier and could not be the cause of the problem. "Having looked into this issue in more detail, it's clear we actually added this parameter a week ago. So whatever happened today to block Google.com.hk must have been as a result of a change in the great firewall," the company said, according to PC World.
According to the AFP, their reporters in Shanghai were able to use Google's search engine without incident, even getting search results on sensitive terms like "Falungong" and "Dalai Lama". For those results, however, the resulting links were blocked, as has been the case. The Shanghaiist also reported two Twitterers, in Suzhou and Shanghai, who reported being able to access Google.
The page tracking Google's sites availability has yet to update to show any change.
It would also seem plausible that even the term "Google" itself is being filtered out by the Great Firewall of China and causing problems using the search engine. When I was in Beijing during the time around the 20th anniversary of the Tiananmen Square Massacre, simple things like advertisements in Yahoo! Mail and oft-seen widgets were showing up blocked and giving errors due to increased censorship.
At the same time, we have to ask - would it really be all that surprising if China had finally decided it had had enough and would put the brakes on the whole situation?