I use a Treo 700p on Verizon with USBModem installed on the Treo. I have a the "unlimited" (5gig/mo) data plan, but no "tether" plan. My N800 has connected flawlessly on OS2007, chinook and diablo. I also use Gizmo on both devices (via WiFi on the N800).

My main use of this arrangement is using Maemo Mapper with my Earthmate BT-20 GPS. I also used this setup the other day to report that my electrical power was out (the Georgia Power outage report page won't work on the Treo's Blazer browser... some stupid, buggy JavaScript!)