Save gas.

When going down a hill the engine runs, the trasmission slows the car down when you release the gas. Point is the transmission gears waste energy in down hiil situations and on the highway. On the highway if I put the car in neutral and let off the gas it will take awhile before I need to apply the gas again case the transmision is not using the the wated energy. When I go to neutral the car still uses gas. If the transmission and engine were computer controled to temporarily turn off the gas and put the transmission in neutral would save gas, rengaging transmission and gas if car slows down 'X' mph. As far as going down hill, the engine off, car in neutral the car should have computer controlled "brake generators" to slow car down(keep from speeding up) and generate elecricity while doing so.
Ken 11/28/2012
Your suggestion saves gas at the expense of wearing your brakes out much faster than needed. When a car is in gear, the transmission helps slow the car when going down a hill. That, and with your idea, if the brakes were to fail, and your car is in neutral, you are in a whole lot of trouble.