Rust On Underside Of Passenger/drivers Door At 3years Old

Have any of you guys had probs with rust on the underside (the part that comes in on top of the sill when the door is shut) of your doors.Cos to my horror I noticed a rusty patch on the underside of my passenger door whilst washing the car a couple of months ago.
I took the car into my local dealer and to my even greater horror they said they had seen this prob before,and they believe it is caused by grit becoming trapped in the rubber seal that runs around the door and then some how rubbing between the door and the sill,thus scratching the underside of the door and causeing it to rust.
Personally I reckon this is bullshit and it's more likely to be a design fault or a bad paint finish.However they did correct the prob by spraying the underside of the door and reluctantly agreed that if anything like this occured whilst I owned the car (regardless of wether it has warrenty or not) they would repair the fault free of charge so I dont suppose I should complain,but rust on a 3yo Jap car is unheard of even on salt laden English roads,thats why we buy them cos they dont rust and are very reliable.

GET IN TOUCH IF YOU'VE HAD THE SAME TROUBLE OR IF YOU HAVN'T GO AND HAVE A LOOK AT YOUR DOORS!!!!!!!!!!!!!!!!!!!!!!!!!