Nope, not a myth at all; there's apparantly a bandwagon of people that are doing it, surprisingly with GREAT results......check gasbuddy.com forums, ecomodders, etc....almost EVERY car forum will have a forum regarding acetone in the fuel LOL.
I have not tried it personally...BUT, I have used products that THEMSELVES contain Acetone...product being Berryman's B-12 CHEMTOOL. Contains a whole witches brew of "High Energy Solvents" including Methanol, Xylenes, Toluene, IPA, and some others....
Suposidely the acetone splits apart the atoms of the fuel to make it "burn better" and thus giving you a better MPG or some such..there's a "wiki" site that started it all somewhere, with real actual "results" and user testimonials regarding acetone in the fuel, what kind of ratio they used....etc. If I recall, back when I did the research it was about 1 oz. per 5 gallons or some such.....
But from my experience with Berryman's B-12 CHEMTOOL, it calls for 1 oz. per gallon of fuel, 1 16 oz. can will treat up to 30 gallons of fuel though....but they say for cars with less than 10 gallon capacity, to add 1 oz. per gallon of fuel.
I did notice a tad increase in performance "in the seat of the pants" lol....likely due to the fact that the solvents make the fuel burn hotter, and thus cleaner....the CHEMTOOL is a VERY potent cleaning agent.....