Reading at the W3C link you made above we have:

This value must be a number between 0 and 32767.

Negative numbers are not allowed. We also have:

1. Those elements that support the tabindex attribute and assign a positive value to it are navigated first. Navigation proceeds from the element with the lowest tabindex value to the element with the highest value. Values need not be sequential nor must they begin with any particular value. Elements that have identical tabindex values should be navigated in the order they appear in the character stream.
2. Those elements that do not support the tabindex attribute or support it and assign it a value of “0″ are navigated next. These elements are navigated in the order they appear in the character stream.

So it would seem that aside from “disabled” no number excludes an element from the tab order. You are correct with zero. Zero moves the element to the last of the tab order. Thanks for pointing this out!

Zero tabindex behaviour is slightly different to how you describe, though this could be an issue of semantics. The ‘zero elements’ will be navigated after the positive tabindex elements. To exclude an element from the taborder (i.e. to make it ‘untabbable’) it should be assigned a negative tabindex.

http://www.w3.org/TR/html401/interact/forms.html#h-17.11.1