I also had my rims replaced at 20,000 miles. It's covered under the original 36,000 mile Body/etc. warranty. Sounds to me like the dealer is definitely pulling your chain. After 8,000 more miles, the rims are already starting to pit! I mean, seriously.... I wipe those things down twice a week. The salt in the winter is what kills them. Next time they Pit, I'm switching to an Aluminum rim. No more Chrome.