Ok, not actually that big trouble after all, although it's quite annonying that the keypad in Ubuntu "Hardy Hedron" 8.04 won't work out of the box (at least for me and some other folks).
For various reasons, I ripped my Hardy install apart and removed a lot of stuff. Currently Openbox acts as the window manager, accompanied by stalonetray, but no panel or desktop (where icons can be put upon) is around. I might get into detail on my custom setup and apps in another post.
So basically, the GNOME user and system customization/configuration frontend tools are no longer around, hence I couldn't directly follow the described steps to turn that behaviour off. Then again, some GNOME stuff is still there, including the gconf-editor GUI tool. Bad memories of the Windows registry come to mind.
To change the setting using gconf-editor, navigate in the tree to /desktop/gnome/accessibility/keyboard and uncheck the key named mousekeys_enable. If it's already unchecked, try to check it and then uncheck it. The result should be instantly notable. My keypad works now (again).
I assume this can also be done using the command-line tools gconftool or gconftool-2.