Here's an answer from a dodge forum
"The other day I was chatting with the dealer about doing the scheduled maint. myself and keeping the factory warranty happy. I asked him why you need to change the plugs every 30000 miles when most new vehicles
nowadays go 100000 between changes. He mentioned that you need to use regular copper plugs in the Hemi, not platinums, hence the reason. I then mentioned that I recently changed the plugs and installed Autolite Plat. XP's and asked if I should be concerned. I was told that the reasoning behind using standard plugs is thatfor some reason platinum plugs have a hard time firing correctly in the Hemi. So what will probably happen is that the truck will run fine for a while then you will start getting misfire codes due to the plugs fouling with the incomplete combustion. The gas mileage will also suffer (I have noticed this, but I figured it was due to going up a size in tire and not recalibrating the speedometer; I guess not). So for now I still have the plugs in and will probably keep them in until they start to give me problems, or I get really ambitious one afternoon and decide to go ahead and change them (soooo much fun). Just wanted to give you fellow Hemi owners a heads up and I was wondering if anyone else had heard about this."