Found this rant on HackerNews, railing against Pinterest's Terms of Service:

If you snap an awesome photograph, upload it to your blog, and someone pins it, that person is either (1) claiming exclusive ownership of it; or (2) giving Pinterest your consent to reproduce it (and you just thought you were being flattered).

It gets even worse:

If some photographer sues you for violating their copyright, Pinterest washes its hands of any and all legal liability.

Does pinning an image to Pinterest fall under the provisions of Fair Use? The answer is unclear.

Here's an interesting post from the American University Law School blog:

Pinterest's Terms of Use are not friendly to users who wished to actually pin their own original content. In general, social media sites require users to grant them a license to use any content posted to that site, but do not make any claim of ownership, and Pinterest is no different.

However, the key provision that is making some people nervous about Pinterest is that its license is irrevocable and perpetual, meaning that even if you delete content, Pinterest can theoretically still use the content however they wish.

In contrast, Facebook's terms state that the "IP License ends when you delete your IP content or your account." Tumblr, YouTube, and Flickr have similar terms.