Fair Use under US copyright isn't cut and dry, to say the least. Columbia University has released a checklist to determine if a use is 'fair' or not.

http://copyright.columbia.edu/copyright/fair-use/fair-use-ch...

Note: I don't think it's been challenged legally yet, but it's certainly a fair piece of policy and a strong guideline, from a professor who is very respected in the copyright law field.