You may want to take a look at this book on the internet.
HIMAC Research - The Secret Super High Mileage Report
This is about "Thermal Catalytic Cracking" which is more than just heating the gasoline. It involves heating the fuel mixture to make it completely vaporized, adding steam and using a catalyst to break up the fuel molecules into other small molecules.
There are some companies that sell fuel heaters for gasoline engines.
Fuel Heater :: Sigma Automotive
According to the EPA, fuel heaters don't save gas.
"Gas-Saving" Products: Fact or Fuelishness?