I guess this relates to XFree86Config -configfile editing..

so check out this one for a full text:
http://www.superant.com/cgi-bin/smalllinux.pl?SmallXFAQ
and here's the shortened text:
when you're running an X session, in terminal run
Code:
xmodmap -e "pointer = 2 1"
if you have a 2-button mouse....if you have a three-button, run instead
Code:
xmodmap -e "pointer = 3 2 1"
if you wish to get it in use in every X-session, create in your homedirectory a file (unless it's already there; then just append) called .Xmodmap and add there this line:
pointer = 2 1
if you have a two-button mouse, and if you have a three-button, then this:
pointer = 3 2 1
and that should do it (perhaps you need to restart X?)
note that if you happen to have a wheel (as many people do these days) or more than three buttons, it's quite unsure how things are done...but you can get a clue from the lines above.