What is the significance of reverse polarity protection

Does the JEtbeam MKIIX have it?
Could it hurt the light or battery if backwards?

I think that info's on the dealextreme web site. If it's got it then it should not hurt the light although I wouldn't leave one in backwards for long. I don't know if it will hurt the battery as it's probably a heavy diode protecting things but probably would not - just realize if your light does not work when you put in a new battery to make sure it's not backwards and be glad your light has reverse polarity protection as they all should have. BTW if it's a diode providing the protection there is probably several Megohms resistance so it should not damage the battery IMO.

Reverse-polarity protection on flashlights is more often mechanical, usually a
recessed positive terminal so that only a cell with a button can make contact.
(WHY don't they put buttons on any of the 18xxx batteries??????)