I am not too sure if this will will fix it but try deleting the usb/ethernet option from NetworkInterfaces.plist, reboot and plug in your usb after leopard loaded up.
1. go to /Library/Preferences/SystemConguration/
2. drag/copy NetworkInterfaces.plist to desktop (so permission will defaulted to read/write for admin)
3. Double click on NetworkInterfaces.plist on desktop
4. Delete the bit where your usb/ethernet info is (from <dict> to </dict>)
5. Close TextEdit. Save.
6. Copy it back to /Library/Preferences/SystemConguration/
7. You will need to authenticate, put your p/w and replace
8. Reboot (make sure your USB WIFI is not plugged in)
9. After Leopard loaded up, plug the USB in and pray to your god or any other that you think might help.
10.Post back on this thread