my p5 rust story goes like this:
my 2002.5 p5 started to rust on the inside of both the rear wheel wells (around the lip of the fender), as witnessed by many IL peeps including ken (captain krm). i figured it should be covered under warranty since it was only 2.5 years old at the time. took it to the dealer and after a week of nagging them they decided they wouldnt cover it because they said it was 'probably' caused by rock chips and other debris hitting the inside of the fender. i was pissed so i went to my trusted body shop to get an estimate. even with a discount it would have still cost over $1000 to fix. so i lost my mind and traded it in for a black 2004 rx-8. i had that for 5 months before i decided that i didnt really want it because it was an auto (blame the wife) and i didnt want to deal with it in the winter (which was approaching). i traded the rx-8 (and lost my ass) in for my '05 3s that i currently have and will own until it dies.
moral of the story: dont let your frustration and emotions get the best of you. if i could have done it all over again, i would have kept the p5 (rust and all) until the ms3 came out.
good luck and i hope it works out for you.