EDIT: Note the last part "affirmed the district court's finding that Perfect 10 was unlikely to prevail on its vicarious infringement claim, because Perfect 10 was unlikely to be able to show that Google had a right and ability to stop or limit the infringing conduct of third-party websites." Pinterest clearly has control over their content and arguably promotes infringement by providing a nifty browser toolbar to do it.
There is also the ASMP, et al vs. Google case that is still pending. See http://asmp.org/articles/asmp-qa-google-class-action.html
The issue with pinterest is the presentation of the original image. Using an image with out explicit consent is illegal even with attribution. Perfect 10 vs. Google revolved around the use of low quality thumbnail representations in search results. Pinterest is not a search engine and does not use low quality thumbnails.
In comparison to Tumblr, it can be argued that Tumblr has a clear purpose as a blog platform above the ability to simply re-blog content from other sites. I am sure Tumblr is inundated with DMCA takedowns as well.
Theoretical lawsuits against Pinterest are more likely to have success because the sole purpose of the platform is to repost third-party content (to what ever means their business model is based on). It will also be interesting to see how well they implement DMCA take-down procedures. Even if the purpose of the site is validated as fair-use, the DMCA procedures will need to be sufficiently vetted to insure they do not lose protection. The recent articles seriously calls into question Pinterest's knowledge and ability regarding handling of DMCA requirements.