Re: ...."not including a phone"....
The iPod Touch doesn't have as high of a resolution 800x480 or as large 4.1" of a screen. The Touch doesn't have a user replaceable battery, upgradeable memory or bluetooth, Flash 9, or support USB OTG. i.e. I can connect an external hard drive, keyboard, camera directly to the N800/N810. Also the N810 has a stylus keyboard, finger keyboard and a slide out hardware keyboard.
You're right "Can't think of a open niche this thing is meant to fill really."
The Touch is a music/video player with web access. The N800/N810 is a portable micro computer.