yes, you need to use a text editor as root to edit the file /etc/XF86Config-4 (if you are using XFree 4) or /etc/XF86Config (if you are using XFree 3).
As far as i know it is as simple as finding the mouse section and changing the bit that says 'ps/2' to 'usb' H O W E V E R i wouldn't bet on it. I know somebody will come on here and tell youi exactly how to do it soon, but i am on my work machine right now (windows 2000
) so i cannot check my own /etc/XF86Config-4 to find out how it's done exactly.