That's a very pertinent question for me as well. I will ONLY use the IBM PS/2 style keyboard (the IBM Model M clicky type) and have one for each of my machines, even my work machine, plus a few spares in case one of these tanks ever fails.
I have adaptors to step the AT (large D) style connector down to the PS/2 size (small D) and I've got adaptors to plug PS/2 style mice into a USB port but I've never tried it with a keyboard.
Maybe it's time to do that. . .
I'm pretty sure that you'd have to use an AT or later keyboard since the original PC keyboards used a different interface. I don't think a PC would recognixe an AT keyboard and I don't think an AT can use a PC one.
Again, I haven't checked those and unfortunately the bulk of my collection is in storage for the time being.
I think you might be refering to a "legacy free" system, like gateway and dell and many others are shipping. These computers have no ps2 ports, so mouse and keyboards must be USB. I have used an adapter at the shop to convert usb to ps2 and then an AT to ps2 adapter so I could use my favorite old keyboard whilst repairing the newer style units. A link to the USB converters can be found here: http://www.usbstuff.com/ps2.html
This along with the AT to ps2 converter worked for me on a gateway with usb only, but YMMV