The maintenance log for my Prius says the spark plugs don't need to be replaced until the 120,000 inspection - unless you live in California, New York, Massachusetts, Maine or Vermont. If you live in one of these states, replacement is not necessary until you hit the 150,000 mile mark!? On all my cars, I've always replaced my spark plugs every 20,000 miles - just on account of, and have always been under the impression that replacement every 20,000 to 30,000 miles was recommended to maintain optimum performance. So it's hard for me to believe that the iridium-tiped plugs that are installed in the 2004 model are going to continue to give me great performance all the way out to 120,000 or to 150,000 miles. Have any of you out there looked into this matter? Your insights and recommendations would be appreciated.