Yep, that was a real sign in Springview, Texas, according to CNN (8/9/13):
In black letters, the signs reads, "No Muslim parking in the Westview Shopping Center. Your car will be towed."
The posters lined the street near the El Farouq Mosque, where Muslims heading to worship services said they were were offended.
I've seen it said that the property owner of the shopping center was trying to post a shopping center customer only parking lot sign, but did so in a, ahem, clumsy fashion. But since the owner denies posting the signs, that doesn't seem very realistic.
When you get loudmouth bullies like Peter King (R-Teh Stoopid) saying:
King expressed his belief that "over 80 percent of the mosques in this country are controlled by radical Imams." link
then you've established the climate where this can happen.