Duke University's Center for the Study of the Public Domain has put together a terrific comic book explaining copyright law and what constitutes fair use. The book is directed at documentary filmmakers, but is an excellent illustration of the concerns that people (such as lawyers blogging) wishing to use copyrighted works for non-commercial, semi-commercial, or transformative uses face. It also is a great introduction, in graphic novel format, to the great cultural debates going on as described in such books as Professor Lawrence Lessig's Free Culture.
I hope that the authors and Duke University will keep up the comic book format - it's fun and contains powerful images that really drill home the practical problems facing authors, artists and educators.
Bloggers should start educating themselves on how to avoid claims that they are stealing the works of others. Tricks like using small snippets and linking back to the author, making sure the author is credited and trying to use as little of the work is necessary, and making sure that the work is commented upon are all not only good etiquette, but may bring your borrowings under the fair use doctrine. Maybe. I'm a lawyer, but I'm not giving legal advice here. As lawyers, however, no one will feel sorry for us if we are caught stealing. Be careful my friends. When sampling busted out in the 80's, it was cool and free. When it started making money, it got shut down. Those who do not learn from history are doomed to repeat it.
On the other hand, if you know your rights and want to take a principled stand, go for it. Just make sure it's creative expression, not laziness and plagiarism that you're going to bat for.