I've tried this on two different USB keyboards - both of which work fine on a different computer running Windows XP.
For some reason, the numerical keypad doesn't work probably immediately after booting up - neither the numbers work, nor the 'home' commands and so on that you can use with the numlock off. It doesn't make a difference whether I press numlock on or off - the keypad doesn't work correctly no matter what state this is in.
However, once I've booted the machine I can disconnect/reconnect the USB connector for the keyboard, and it will work exactly as expected.
I'm running Gnome on Ubuntu 10.04.
The only other USB devices connected is a mouse - and I've experienced no problems with that.
This is a direct connection to the box (not via an external USB hub)