For ergonomic reasons (discussed previously) I like a side button on my mouse to act as a middle button instead. Unfortunately there still doesn't seem to be an "easy" way to remap mouse buttons in Linux, and what's more the method of changing such settings seems to keep changing. To achieve the same results in Ubuntu Lucid, I added an Xorg configuration fragment at /usr/lib/X11/xorg.conf.d/20-logitech-mx1100.conf:

Section "InputClass"
        Identifier "Logitech MX1100 button remap"
        MatchProduct "Logitech USB Receiver"
        MatchDevicePath "/dev/input/event*"
        Option "ButtonMapping" "1 2 3 4 5 6 7 2 9 10"
EndSection

As with the previous method, xinput list should give you the product string to use for MatchProduct.