I reserve the right to be dead wrong:

There's no way a USB EVDO modem will be able to power itself off of the N800's USB-OTG port. The N810 may provide that much power, but it's extremely unlikely.

Once USB-OTG catches on, I would expect to see compatible self-powered modems. USB-OTG is useful for low-power or no-power devices only --- it gives you host mode and gives you that little bit of power you need to activate the client device.

Read up on the 770 Host Mode docs for more details.