In some cases, for instance a laptop with a built-in pointing device,
you may wish to use a serial mouse as a second device. In most cases
the built-in device uses the PS/2 protocol, and can be ignored if
you don't wish to use it. Simply configure gpm or X to use /dev/ttyS0
(or whatever) as usual.
To use both at once, you can use gpm -M to re-export the
devices. More details in the gpm man page. Also, XFree 3.3.1 and later
support muliple input devices, using the XInput mechanism. Auto-generated
XF86Config files should have the necessary comments in them.