Is there a way to tether a cell phone to a Routerboard USB Port to use the cell phone as a cellular modem?

probably easier to use the many "usb sticks" provided by most carriers

shouldnt be much different. just plug it in, you should see a serial port or two show up. Configure a PPP async adapter on it and it should be good to go. You might have to monkey around with the connection init string or phone numebr (#777 for verizon).