Is the USB keyboard directly attached ? Did you try a different one ?
vmkernel log should mention if the keyboard was discovered.
Is there a BIOS setting for legacy keyboard emulation ?
Thank you for the suggestion. I just finally solved the problem a few minutes ago. It appears that the VMware hypervisor does its own IO to the USB keyboard, without going through the bios (there are no PS/2 ports on the dell servers) and may not find or work properly with all USB ports on the system. I had previously plugged the keyboard into a USB port on the back of the server (which worked fine when I booted linux). When I connected the keyboard to a usb port on the front, it worked properly.