[GZire]

I agree that your aftermarket install is probably the cause for the emissions warning. I thought that there was a sensor by/in the headers/cats that monitored this sort of thing.

If the sensor was not replaced correctly/reset, then most likely that is the reason for your warning light.

The dealer might give you crap, but keep this in mind:
1. They cannot void a warranty, only a manufacturer can and then only after proving your modification is somehow causing damage to the product.
2. Give the service manager a call before you go in to feel the waters. Regardless of what he/she says, you'll have to take the car in sometime and switching out your headers/exhaust is not an option (too much money for labor).

Good luck and let us know how it turns out.