Why the N78?
(I assume you said N78 to say buy a phone that can act as a modem, but if the N78 has something special, I'd like to know)

Using my phone as a modem is out of the ToS of my provider, it works, it's free, but it's forbidden in the ToS.
A SIM card slot in the NIT would just bring me back into the legal way.

And it looks like I'm not the only one:
http://tabletblog.com/2008/07/users-...t-3g-n810.html