Here in the UK, Allisport do a propane kit (which is in fact, imported from the US!)
IT's very popular in Malaysia on their off roaders, helps power no end with little in the way of negatives. The only one i've heard of, is over use can cause the bores of the cylinders to get "dry" (as in, the gas causes 100% of the fuel AND oil to be burnt... oil that the scraper rings havn't totally cleared that help to lubricate the piston) so you can get excessive bore wear.
However, I think it's Mercedes that offer propane injection on their heavy trucks, so it can't be all bad!
Good thing about it too, is you can crank up the fuelling for more power, but not get the clouds of black smoke, as the propane makes sure all the excess fuel is burnt.