Actually, the N900 has been "on again, off again" at the NokiaUSA.com storefront for some time now... for that matter, NokiaUSA.com is a joke - it's basically a disjointed e-commerce site, with only a hodge-podge of phone info, and most of its support options cross-linked to europe.nokia.com.

You can still get the N900 from Amazon, and NewEgg still has some stock left (correction, I just checked and they're out).