Possibly the moisture indicators are positive, but that alone is no grounds for voiding the warranty.
If they tried this stunt in Australia, I would take them to small claims court (no lawyers allowed), with plenty of evidence from this forum.
Unfortunately, I imported it from the US, so I'm at their mercy.
Is there any practical way for consumers to take sellers to court in the US over warranty disputes?

edit: actually, I think you have to take the retailer to court, for a statutory warranty claim. And statutory claims can be made later than the 12-month Nokia warranty lasts. Probably at least 2 years for an expensive phone.