I predict that the N900 will continue to be able to be used as a phone and a web browser and for many other functions for many years. If 3d cell phones are developed, or phones that are invisible and wired into our brains, I predict that almost none of the phones that you can purchase will work well with the new capabilities.

So, if you want something that will last into the unknown, indefinite future and has the capability to remain at the cutting edge of whatever happens next, I think you are out of luck.

On the other hand, if you want something that continues to be usable for what you can do with it now, I think you are pretty safe.