Just an update for you if anyone looks. At the moment the new law for Florida has been revoked as its in breach of the Geneva convention which the usa has signed. Florida are still planning to try to push this through but for now its on a backburner. This is the most recent information I've had, I work for Tui and we received this update yesterday. hth.
IIRC you do not need an international driving permit for driving in the US if you already hold a full UK driving licence.
From the Foreign Office website re the US:- Passport validity Your passport must be valid for the proposed duration of your stay; you do not need any additional period of validity on your passport beyond this. However, it is always sensible to have a short period of extra validity on your passport in case of any unforeseen delays to your departure. You do not have to wait until your old passport expires to apply to renew it. Any time left on your old passport when you apply will be added to your new passport, up to a maximum of nine months
We're off to Florida in May and have never bothered with an IDP for hring a car on our previous US visits (to other States though - Massachusetts and California). There's conflicting info as to whether they're required or not - does anyone know? Also, DD's passport expires in November. I've read that there doesn't need to be a particular amount of validity left on the passport in order to travel - has anyone had experince to the contrary?