Clear Channel issued a statement blaming their local office. The original story (which Prison Planet directly copied) has been updated and includes the full text of a letter published in the Orlando Sentinel complaining about the sign; the Sentinel editor quoted by name says they wouldn't have printed the letter if it weren't based on a real sign. The letter is indeed on the Orlando Sentinel's website (without registering, you can see most of the first paragraph complaining about the billboard and it's the same as on Blue Lemur's site). Sounds legit, but it'd be nice if it appeared on more mainstream news outlets.

Reminds me of this heavy-handed toon from a couple of years ago: