Perfect 10, a company selling nude model photographs, has launched a lawsuit against Tumblr for failure to respond to DMCA notices. It's likely that Tumblr will respond by taking the content down, and pretending it didn't receive the notices, or that the notices were incorrectly formulated.
There is, however, only so much that can be hidden under the safe harbor provisions of the DMCA.
The courts will have to settle the matter.
Can you provide your users with a one-click copyright-infringement tool (the pinmarklet), and still hide behind the safe harbor? Can you receive thousands of DMCA take-down notices from a single user, without being pro-active, and still hide being the safe harbor? Can the mother of the co-founder have 15,000 possibly infringing images, and still hide being the safe harbor? Can you keep accepting pins from Google Images (by definition, nearly all should be infringing), and still hide behind the safe harbor? Can you force copyright holders to chase every pin, when they could more easily report the one image the pins and repins are associated with, and still hide behind the safe harbor?