GH1: Actually, Clint Smith of Thunder Ranch has shown people how to use a fire extinguisher in just that manner and it will work great if you hit them hard enough with it after foaming their faces. It is best to use it in a 'buttstroke' application, becase if you swing it at them it is liable to drop from your hand when you bean them with it....
Ariel: two popints. There is a fellow teaching self defense in Minneapolis, I believe and he also uses Wasp spray and advocates it as a 'non-deadly alternative'. And, William Layton is quite correct - Wasp and Hornet Spray is very, very flamable and the technique is to use the spray to first stop or delay a attacker or two and then if necessary, flik your Bic to heat up the situation. These sprays advertize a 20-30 foot spray and if you have ever used that stuff you will know it has deadlier chemicals than just detergents to cause wasps/hornets to lose their grips and fall down - the stuff is instantly debilitating. And yes, adding fire to the fuel is pretty midevil but so is attacking you with a knife or club..... jmtcw.