About the killing app . Skype is by no means the killing application. For me the killing application is the phone-editor, that is, a PIM application that doesn't even have a database on it, because it basically edits what's on the phone.
A phone layer I think they call it.
So you wanna call someone? Ok, you don't need to stop using your N800. Simply use bluetooth to connect to your already synced Nokia Phone, doubleclick and there you are, calling them with your phone but using the N800 as bluetooth speakers emulation.

Linux has all the protocols there. The phone too. You just have to make them work flawesly with N800 and there you are. You can have full control of you phone without taking it out of your pocket.

Everything is there for Nokia to use: PIM-bluetooth protocols, File protocols so you can see the pictures you take with your phone, TCP/IP over bluetooth (that is already working), Phone calls over bluetooth.

That qould be the killing app, because it would mean N800 would become an extension of any Nokia phone, so Nokia could push it to every Nokia customer out there.