In GNU Emacs 23.0.60.24 (i686-pc-linux-gnu, GTK+ Version 2.12.9)
 of 2008-12-15 on escher
Windowing system distributor `The X.Org Foundation', version 11.0.10400090

When I detach the tool bar, what looks like a one pixel gap appears in
the bottom border of the menu bar below the `F' in `File'; here is an
image (made from emacs -Q):