I think this has merit. I do wonder how hard it would be to impletment as you would need to have multiple pumps while the transition was under way. This might make your idea cost prohibited.

It would be simple enough to slip a plastic clip over the nozzle of a distinctive color saying "diesel" or "regular" or whatever.

Modifying the physical shape of a fuel nozzle and inlet is a bad idea; you are right, it would be very expensive.

I would retrofit a small RFID chip (<$0.10) onto a fuel resistant adhesive package (1/2 the size of a dime or smaller) that could be purchased at any Auto Parts store. Then require all gas pumps (diesel nozzle is already too big to fit in gas inlet) to be retrofited with an RF transciever at the nozzle so that for manual pumps an alarm sounds, and for digital pumps the pump shuts off and gives an alarm message, which can be over-ridden and automatically documented in the pump history by the customer.

http://www.smartcardalliance.org/alliance_activities/rfid_FAQ.cfm

This way it is up to the customer to retrofit their diesel vehicles and equipment, but it will be inexpensive enough for them to do so easily.

The main expense will be at the gas pumps. The gas companies could be mandated to provide a larger margin of profit for each gallon sold (currently $0.08 / gallon) by fuel stations. For example: A station purchases 1000 gallons of gas from the oil company. They are given $0.18 / gallon sold instead of the current $0.08 / gallon. That's $100 extra per tank purchased. The oil company tracks how many gas pumps the station has and how much fuel is sold to that station. Let's say each pump costs $60 to retrofit (reasonable considering the large scale of purchases and modifications). Then a station with six(6) pumps only has to refill their tanks 4 times to cover their costs and for the oil company to revert to their $0.08 / gallon payment to the station.

This would also allow for future custom fuels to take advantage of the same system, like: fuel oil, aircraft fuels, kerosene, hydrogen peroxide, vegetable oil, bio-desiel, ethanol, ... If you have a vehicle that can not take anything except a specific type or grade of fuel, you can lock all other fuels out by having the right chip mounted near your fuel intake.

Currently only diesel vehicles can get the wrong fuel; which includes farm equipment, construction equipment, diesel generators, portable fuel tanks, and the smaller fuel cans. There are separate pumps for cheaper off-road diesel fuel, but in a pinch a contractor will buy diesel from anywhere they can get it.

The least expensive change would be to leave things as they are, since contractors and diesel automobile owners are well aware of the consequences; which are rare; but potentially catastrophic for large diesel engines.

In Canada anyways, the nozzles typically have splach rings, and a plastic cover over the handle. Diesel pumps/nozzels have a yellow handle/splashguard.

I don't believe this is a very common problem and probably not worth this effort.

Usually people that fuel vehicles know what fuel their vehicle takes and they know how to read.

In my area, diesel pumps are supposed to have a green handle--except BP has gone and put a green handles on all of their pumps.