I wonder how Tumblr handles this. Pretty much every Tumblr blog I've stumbled on to contains full size copyrighted images re-hosted by Tumblr.
I don't use Tumblr, but my understanding is that a user has to upload an image for Tumblr to host it. The DMCA safe harbor provisions apply in such cases. Pinterest itself downloads images from pinned items, which probably makes them liable for the infringement.
I can imagine content creators being unhappy that their image is posted to some blog, and then pinned without their name. People seeing the pin may (but probably don't[1]) click twice to get to the blog, but how many of those people then click again to get to the creator of the content?
To me, the artistic pinboards are gently problematic. The product pinboards are less troubling, because I guess they drive some traffic to the selling sites.
[1] I have no data for this. I realise I might be wrong.