Unplugging a PS2 device while the machine is on can technically cause a short, even though most motherboards, especially newer motherboards, are protected against this sort of thing.
You did all this troubleshooting with the IPAC disconnected, right?
What happens if you plug the IPAC back in, but with no keyboard attached? Does the IPAC work?
You said you're using a PS/2 keyboard.. Is the IPAC USB or PS2?
Does a USB keyboard work?
It works in the BIOS, you say. How do you know? Did you go into the BIOS and mess around, or are you assuming it works because it doesn't beep during the POST?
Just some things to check...
-Steve