One thing for site owners to be very careful about, if they chose to let Pinterest run against their sites is "who actually own your images"..
If you took that photo, or created that graphics yourself, then you should be safe.
But if your images are merely licensed from someone else, such as when buying stock-photos, image packages with their typical EULAs, or product images under a license from the producer, specifically enabling Pinterest (such as adding their pinning widget) can put you right in the middle of the quagmire.
Affiliate type sites could have that exact same problem. I would advise that people read the affiliate contract they have with each "advertiser" they signed up with to make sure they allow such use of product images. Many of them are getting very restrictive.
The same second the Pin widget goes up, that site most likely becomes just as liable as Pinterest. Pinterest is liable because they provide both the tools and their bots actively do the heavy lifting of taking images as "ordered" by their user. (Thats the difference from Facebook and such, BTW. Pinterest do not fit the Safe-Harbor clause under DMCA. They are not innocent bystanders to bad user-content.)
There is a reason Pinterest is now looking at PicScout's (Getty Images') fingerprinting software for some initial protection. Its because they know they are moving into legal doodoo-land and their business model has the potential for falling apart. Just like Napster.