My father had a neon from that generation and had all kinds of problems with carbon build up on the heads. Had to have it serviced under warrenty 3 times, and finally sold it before the warrenty expired. Perhaps your acetone is helping keep the engine clean.
On the milelage issue again, although I'm sure you don't drive highway exclusively, 34mpg is within the epa estimated range for that vehichle, tranny combination. Although I don't doubt that the acetone has probably helped keep the injectors etc. clean, and you might not be still getting that mileage otherwise, I wonder if you switched back to straight gasoline if you would see a change in mpg or not.
Also, although I find it somewhat suprising that you havent had any issues with seals, etc., your comment makes me wonder. Although your vehichle is not (at least to my knowledge) flex fuel, I wonder if flex fuel vehichles would be better suited to run gasoline blended with acetone since they are designed with fuel delivery systems resistant to the drying (somewhat corosive, at least in regards to rubber) effect of ethanol. Food for thought anyway.
Mods: Thule Xsporter 422XT bed rack, Underrail Rhinolining, MOPAR class IV Hitch, DIY CAI, Aero Turbine 3030.
Future Mods/Wishlist: Tuner, Daystar Comfort Ride 1.5" Leveling kit, Nerf Bars, Police style push bumper with light guards, KC or Thule trail lights, HID projector lights.