Update: This law was repealed on April 2, 2013. See Florida International Driving Permit Law Repealed.
Effective January 1, 2013, visitors to Florida with a foreign driver's licence, including Canadians, must also carry an International Driving Permit (IDP) issued by their country of residence. The Florida Department of Highway Safety and Motor Vehicles says the new law applies to drivers of all vehicles, including rentals.
According to the Florida highways department, the law was brought in to help Florida law enforcement deal with licences in foreign languages. In the next legislative session Florida plans to amend the law so that foreign licences in English will be permitted, but until then Canadians should carry both their provincial driver's licence and an International Driving Permit.
An IDP must be issued in the same country as your country of residence, so you need to get one before you leave on your trip.
You can get an International Driving Permit in Canada from the Canadian Automobile Association (CAA) for $25.