Nikon didn't want to service "gray market" goods in the US under warranty. They got tired of doing this by serial number and decided to use model numbers to indicate goods officially imported by Nikon US and those purchased outside the US and imported by someone other than Nikon US.
No US model number means no US Nikon warranty repair regardless of purchase date.
Quote Originally Posted by PhotoJim View Post
I'm not sure why it's the N90s in the US - it's the F90x everywhere else.