Listing only top level pages (Yeah I read the Codex!)

I am using WordPress 2.7 right now and I have a problem listing only top level pages using wp_list_pages.

I used the standard code to exclude some top level pages being listed. Example:
wp_list_pages('sort_column=menu_order&exclude=2,3,4&depth=1&title_li=');

One page has a subpage. When I exclude all top level pages from being listed this subpage is being listed, even though I set the parameter depth=1.

As long there is one top level page being listed (not excluded) the sub page does not appear.

Is this done on purpose or is it a bug? Or am I doing something wrong?