Which ever way you take it: you don't take either the N800 or the E90 on the beach or skiing, or clubbing. Whereas you still need a small, light phone in those precious moments (would it be to call a cab).

Which means that you have to leave your dear device at home /office. If you belong to these families of clients that can't get 2 sim cards for the same number, then why not take the N800 and a small 3G phone with HSDPA and decent camera.
Otherwise, you have to get 2 phones (the E90 and whateverelse). The total price gets high...

Of course and this is where the problem arises: IF you can you use the N800 for minimum business applications (PIM etc, there is a 10 mile long thread on this).

So in a way yes the N800 would become a competitor to the E90.

For me, the fact that the N800 isn't a phone is a selling point, because I don't want a monthly charge and I already have an emergency cellphone anyway. People say they can't get useful work done on the N800, but I have had several jobs that depended heavily on reading web pages and sending email. I can do them just fine with my N800.