Naturally Miami's own 2 Live Crew may have been the brave legal scholars that could have helped blazed the trail for a win for Fairey.
Back in the '90s the Crew made a parody of Roy Orbison's hit "Pretty
Woman," called "Oh, Pretty Woman." The case, Luther R. Campbell aka
Luke Skyywalker, et al., Petitioners v. Acuff-Rose Music, Incorporated,
made it all the way to the Supreme Court.
The Rehnquist court ended up ruling unanimously in Uncle Luke's favor.
Unlike
the 2 Live Crew song, the Hope poster was not a parody, but the ruling
may still be used to influence the outcome of the case. The court found
that a parody does not diminish the commercial worth of the original,
and it would be hard to prove that Fairey's derivative use of the image
diminished the commercial worth of the original. In fact, it's probably
elevated the original's worth (copies of the image are being sold in
art galleries).
For a more in depth discussion of the topic check out this Wall Street Journal article and today's Fresh Air with Terry Gross.
Edit: Of course, the 2 Live Crew case is far from the only decision that could impact the Fairey lawsuit. For an explanation of a more relevant case involving Jeff Koons please see this comment.