It really depends on how far you go; I don't see clipping bits of text, esp. broken up, as an issue. If I use part of a poem or quote in a painting or collage, I always include the authors name on the actual piece. Photo clippings are a whole other issue. If say you are tearing up magazine photo's for bits of color or a tiny object that's one thing. If you are using a large section of someone else's photo, you could get into trouble. Certain corporations, Disney in particular, has gone after small artists who use their icons. I have also seen people to 'mixed media' where they are using very complete areas of other people's actual art work, completely unethical. Even if I paint from someone elses photo, I ALWAYS ask permission.
Hope that helps a little. There are ton's of sites online that have copyright free images. I usually only use magazines for fragments of color or pattern.
Here is a discussion we had a while back:
Helpful articles: http://www.artbusiness.com/copyprobs.html