Flickr users in China began reporting that access to images was blocked on June 7, and the ban appears to still be active as of June 11.
A Flickr spokesperson says, "Our technical staff have looked into this and determined that it's not a technical issue from our end. Evidence suggests that our image servers are being blocked for many users in China."
Here's the support forum discussion on Flickr, and The San Francisco Chronicle has a related story here. See also this CNET item, this Slashdot thread, and this Shanghaist item.