Last April (2012), I rented cars in Belgium, Rome, Inverness and London (3 separate hires). Not once did any of the Hertz agents ask for a IDP. In May (2012), I hired a car in Orlando and a RV in San Franciso - again, no request to site an IDP.
The RACV web site (International driving requirements - RACV Travel) says that Belgium, Italy and the USA all require an IDP. In my prior experience with car hires in France and Spain (where it's required according to the RACV site) I have never been asked for the IDP.
Being asked for it, and being legally required to have one are seperate issues. The us hasn't required one previously (from recollection), perhaps the Florida kerfuffle changed the advice on having one.
The DMV/licensing/roads authority in the country or state where you want of drive should have information hidden away also.