For me, main differences are (if not counting the major pocketability and on-the-go factor):

1. Battery. I want the thing to be on (pidgin, mail, etc) and at the same time not something I can trip over in my livingroom or keep in mind when to recharge. My N810 can survive at least a morning-to-night cycle, I'm skeptic x86 netbooks can do that.

2. Boot/suspend time. Often I find myself looking up things on wikipedia because of a mention in a TV show. Previously, the boot/wakeup time was enough deterrent to forego doing that on my notebook.

3. It's the same device I take with me. I don't have to sync back and forth with a third or fourth device.

4. It's not the same price range, not nearly.