...And I would be very wary of introducing tabindex at all.
I have a navigation bar which is assigned a tabindex value, so that it is possible to navigate through it using the keyboard.
Focussable elements are already able to be accessed via the keyboard, naturally in source order (except in Opera where you can choose a spatial direction and are not limited to source order).
Tabindex is really for when your source order is unavoidably weeeeeird and you'd like the keyboarding users' experience to make sense. Otherwise, try NOT to use it. It can seriously screw with a keyboarder's ability to navigate where they want to on your page.
I'd be too scared to try it on a real page.