Supplies of
good non-USB PC keyboards are probably beginning to get harder
to find now too...
True. It could use either kind of keyboard. Also, USB to mini-DIN adapters are still
sold ($5 in the current L-Com catalog). That seems like a cheaper solution than a USB
Host shield.
If you mean one of these, they are just passive adapters that only
work if the USB keyboard (or mouse) is capable of supporting both USB
and PS/2 interfaces.
http://www.l-com.com/usb-usb-adapter-type-a-female-mini-din-6-male
I imagine the majority of cheap disposable keyboards manufactured now
are USB only devices, not combination USB - PS/2 compatible devices