These days most mice have a scroll-wheel that doubles as middle click. They're not bad, but I prefer the left and right mouse buttons. So why not remap them?
On my Sun Ray I run the following:
% xmodmap -e "pointer = 1 3 2 4 5"
I think the syntax is very clear... 5 logical buttons are assigned by listing the physical buttons in order. i.e. the default is "1 2 3 4 5" but as I want to swap buttons 2 & 3, I transpose them in the list