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.