Automatic Menu Structure

Hi,

In the old themes, such as twentyten, the structure of the theme’s main menu would automatically reflect any movement of pages. For example, if I have the structure

About
– Company History
Projects
– Project A
– Project B
– Foo

If I were to edit Foo and change its parent from ‘Projects’ to ‘About’, the menu structure would also change to reflect this. In recent themes, this is not the case.

Does anyone have any idea why this is, or even better, how to bring back this functionality? I’ve looked in the functions.php file of twentyten but it doesn’t seem to be there. I could write a custom function to do this, but we’re quite time-limited at work right now and this is for an ad-hoc customer request rather than a pressing issue.