Using serial port for a WiFi connection

Quick question - I want to connect my Arduino NG (USB) to a WiFi module which has all the TCP/IP etc, so I can send email statuses from my controller. http://www.tdc.co.uk/index.php?key=wism

BUT - the serial port is already committed to the USB port, I guess I have to power the board separately from an external PSU so that the USB chip is not powered up, otherwise it will conflict with an external serial device?

Is that right?

TIA, Simon