I also wrestled with this issue, both debating with myself over whether to get it, and also looked for feedback here. My dealer offered it as an "environmental package" (rustproofing, paint sealant, fabric stain guard) for $400. There seems to be no clear answer on this issue (can't find anybody here to say "if you don't get it, your car's going to rust" or to say "it's a total scam and it does nothing") but the majority opinion I've found is that it's mostly unnecessary. Comments I've found against it usually cite better materials and processes used at the factory now on newer cars, the many loopholes within those rust protection plans, and the possibilty that the very application of this stuff could, if done wrong, actually harm your vehicle making it more prone to corrosion than before.
I decided not to get it. I'm betting that, if I keep it washed regulary, ridding it of road salt and debris, and keep it protected with occasional waxing and garaged from the elements, I'll be okay for a long time. The protection the dealer is trying to sell you is, no doubt one of the highest profit margin items they've got, so keep that in mind while they're offering it to you.
Good luck with your decision and your new Odyssey!
'02 Ody EX SS
'99 CR-V EX