From Thomas Hawk:
Well unfortunately it looks like the Chinese government may be censoring Flickr images in China. There are not a lot of details on this yet, but Flickr Chief Stewart Butterfield has issued two comments on a help forum on the matter:
Update from Flickr staff (10:00 PDT, June 7th) : It seems that access to our image servers is being blocked for users in much of China. Our technical staff has looked into this at depth and determined this is not a technical issue from our end. We will keep an eye on the situation and update if we get any developments.
Update from Flickr staff  (01:00 PDT, June 8th) : We are checking periodically to see if the block is still in place, but haven’t detected any change. We hope that this is a temporary issue and we currently believe that it will be. In the meantime, we are investigating our alternatives. Thank’s for your patience,
If you want to follow this case on an ongoing basis you can follow it in this official help thread over at Flickr here.