Many mobos do not do proper USB->PS2 emulation (some don't do it at all) so if all you have is a usb keyboard, you're stuffed. You can't tell grub which option you want so you get the choice of default, default or default. Tough luck if your upgrade broke but you made that the default in order to test it ...
For example, my mobo is broken some of the time - kernel upgrades are a pain because I never know whether my keyboard is going to work with grub or not :-(