I have a Dell Optiplex 390 at work and I've had a strange keyboard issue I couldn't explain: my USB keyboard simply refused to work until my OS was up and running. Whether it be Windows, Linux, or FreeBSD -- no keyboard functionality until the kernel had initialized the USB devices.
My keyboard is an Adesso MKB-135B -- a nice keyboard with Cherry MX Blue switches. My coworker with the same computer has the same issue with his Logitech somethingorother. I'd always have to pull out Dell keyboard if I needed to modify BIOS settings or do something in GRUB / BSD loader. It was quite annoying. I'd tried several BIOS updates with no luck, but recently I was annoyed by this bug again and checked for another BIOS update.
Good news to anyone googling this: as it turns out the latest BIOS update -- A10 -- actually fixes this issue!
My theory is that this bug is a power issue -- it wasn't putting out enough power for these keyboards. I never bothered trying to verify it, but maybe someone out there will.
edit: This did fix the USB-on-boot problem, but I have since had problems where I come into work in the morning and my keyboard won't work. Unplug, replug -- no go. Try different keyboard and it works. Plug my original keyboard to any other computer and it works. Very strange; the only fix is a power cycle. I've updated to BIOS A11 which I hope will fix this issue.