I also don't buy the copyright infringement argument. Pinterest is a bookmarking service much like Delicious. It no more infringes than the thumbnail previews in Google Images or Facebook posts.
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.