I am running Debian Lenny. My system was operating normally, but after
rebooting the computer, my X11 mouse pointer no longer moves in response to
mouse movement. I am using an IBM compatible computer, and my mouse is
attached to the PS/2 port on the back of the machine.

After encountering this,I unplugged the mouse and re-attached it after
testing it on another machine. The connection is a good fit and I suspect
that the problem is being caused by a configuration change that has
occured maybe as a result of an upgrade by Debian.

Is there some sort of diagnostics I can run to see if there is communications
coming from the mouse? Could I tell via software, if there appears to be
a mouse connected to the PS/2 port?

My mouse operates via the /dev/input/mice driver.

Mark.

You can type "sudo cat /dev/input/mice" and move the mouse.