Instead of rehashing what has been said already, I have listed some great articles that discuss this issue. I strongly urge you to read these posts.
Not Pinining for pinning
by Love Life
NB: This post also has a great list of articles on this subject.
The Ugly Truth about Pinterest
by Boutique by Design
Don't pin me Bro! The Saga of Copyright and Pinterest
by Aberrant Crochet
Pinterest Users need to read the fine print by Technorati
Also check out LINKwithlove.
Thank you Kellie Wishart for initially bringing this issue to our attention.