Following on from Bob's admin note, here's some background to fair use laws in the US:
"Fair use is a copyright principle based on the belief that the public is entitled to freely use portions of copyrighted materials for purposes of commentary and criticism. For example, if you wish to criticize a novelist, you should have the freedom to quote a portion of the novelist’s work without asking permission. Absent this freedom, copyright owners could stifle any negative comments about their work."
http://fairuse.stanford.edu/Copyright_and_Fair_Use_Overview/chapter9/
"The four factors judges consider are:
•the purpose and character of your use
•the nature of the copyrighted work
•the amount and substantiality of the portion taken, and
•the effect of the use upon the potential market."
http://fairuse.stanford.edu/Copyright_and_Fair_Use_Overview/chapter9/
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