TheMadScientist - 6:46 pm on May 21, 2011 (gmt 0)

I personally find this interesting:
(Emphasis Mine)

4. The effect of the use upon the potential market for, or value of, the copyrighted work ... Acknowledging the source of the copyrighted material does not substitute for obtaining permission. ... When it is impracticable to obtain permission, use of copyrighted material should be avoided unless the doctrine of fair use would clearly apply to the situation.

See Preceding Reference to CopyRight.Gov for Source and Additional Information.