Finally got a photo of this amusing sculpture on the Ponte Alle Grazie by the French artist Clet Abraham. He has been making street signs into art around Florence for a while (I have a whole bunch of those photos I need to post) but this piece is unique and quite bold as it appears to have been done without permission. It has been in place for about a week now I think - we'll see how long it lasts!
You can find more about the artist from these links (his studio is in San Niccolo):
http://www.turismo.intoscana.it/allthingstuscany/tuscanyarts/street-signs-clet/