Dan,
I think it has pretty much been said, the thunderstorms at this time of year in Florida and frequent. However, I have been to Florida in August flying on more than one occasion, and it does not really cause a problem. As has already been mentioned, if you fly early in the morning, through to early afternoon, it should be fine.
Another thing is that the thunderstorm activity tends to be islolated and you can see them forming. When you see all those ing Cb's you know you are in for some heavy showers and lighting!
Enjoy it in the states, it is real good fun.