Child Theme Redeclared Functions do not behave as Codex says

Sorry for double post. Only just discovered this forum:
In the codex, where it explains the new features of WP 3, it says:

TIP FOR THEME DEVELOPERS. The fact that a child theme’s functions.php is loaded first means that you can make the user functions of your theme pluggable—that is, replaceable by a child theme—by declaring them conditionally. E.g.:

if (!function_exists('theme_special_nav')) {
    function theme_special_nav() {
        //  Do something.
    }
}

However, whenever I try to redeclare a function in a child theme functions.php that is already declared in the parent theme, I get a fatal error ‘Cannot redeclare….etc’.

This happens whether or not I wrap the redeclared function in
if (!function_exists

or not.

Is the Codex wrong?
Should I report this as a bug for WP 3?
Or have I misunderstood?

I am running WP 3 Beta 2.

Many thanks.