Who's to say you can't have a N900 a truely powerful mobile device with you everywhere, and a Nslate to bring with you for more mundane things. I think N900 has the potential to be able to do just about anything, but one thing it can't do is have a bigger screen.

Sure I can watch a movie or read a webpage on my N900 very well and anywhere! Like in a bus I'll whip out my N900 and watch a movie. It's not like I'd whip my Nslate out in a bus, unless it's a really long bus ride :P
Even in the morning I enjoy reading a paper and browsing the net. I could do all that on my N900, but I sit infront of my desktop computer to do this. But if I had a Nslate, I'd sit at my dining room table together with my gf and have the Nslate there with me.

It would be a luxury item. A inbetween item, that you don't actually need, but would be awesome.