I'm not sure Leopard incorporates appropriate drivers to see the phone as a USB modem. That said, there is a program called USBModem by Mobile Stream that I used to use to tether my phones to my wifes G4 running 10.3.9. I'll see if I can find a link. It's not free though, so if you want don't want to pay you will either have to do it over BT or find a version of WMWifiRouter .91 (all over the net, probably in posts here and on the FTP too).
Edit: Link.
http://www.mobile-stream.com/usbmodem_wm.html