I was having a lot of trouble with PS/2 in 10.5.x, and thanks to Chun Nan and others, I got a solution. So I put the right kexts into an installer package so it will make it easier for others. The package is attached below, and 10.5.4 is required to use it. Some of the files I used: AppleACPIPS2Nub.kext and ApplePS2Controller.kext by Chun-Nan (based on DFE's ACPIPS2Nub.kext). I've tested this package on my own system and it works fine, but if you have issues, please feel free to tell me. Many thanks to Chun-Nan and the others that helped me get my PS/2 working!
If you have a PS/2 Keyboard or a PS/2 Keyboard AND Mouse then install the PS/2 Keyboard package. If you have only a PS/2 Mouse then install the PS/2 mouse package. Don't install both!!
EDIT: And another thing, please leave feedback here if it worked for you