Saw this linked from Slashdot today. It's a writeup of an experiment in ISP censorship. Basically they posted a section of Mill's On Liberty on various ISPs around the world, and then issued a copyright complaint against it to each ISP. Since the text was written nearly 150 years ago it now falls in the public domain and it is therefore perfectly legal to publish it in this way. The response from the ISPs varied a lot, from immediate censorship to a full and complete investigation. Read the article for full details.
The author argues that the governments have taken the easy way out by placing responsibility for policing this content on the ISPs.
While it may be the easy way out, as far as I know it's the way it's done in the printing industry. The printer is as responsible for checking that what they are printing. ISPs (and probably printers but I'm not sure) protect themselves from the potential costs involved in legal action relating to copyright violations by including clauses in their Terms and Conditions that expressly state that they can (and probably will) sue their client to recover any costs they incur due to such legal action.
I certainly think it's unreasonable to expect ISPs to check every item published on their servers before they are made available to the general public. I do believe that the current system that requires that ISPs react to and take appropriate action regarding notifications of copyright violation can put ISPs in a difficult position when it comes to unfounded notifications.