The visual cue to the user that a menuitem launches a submenu is a right
arrow.  When the user selects that menuitem, the submenu appears to the
right of the parent menu.  This is good.

However when placing the submenu to the right would cause it to go off
screen, the submenu is placed to the left.  However the visual cue in the
parent menu still is an arrow to the right.  This arrow should be flipped.

Most troubling is when the user navigates the menu using keys.  When the
submenu is placed to the right of the parent menu, the user enters the
submenu by pressing the right arrow key.  Logical, since the cursor must
move to the right.  However, when the submenu is placed to the left of the
parent menu, the user sees the new menu on the left, and presses the LEFT
key to enter it.  (Afterall the cursor needs to be moved left.) 
Surprisingly, this causes the toplevel menu to be changed to the previous
toplevel menu.  Instead the user must press the RIGHT arrow to move the
cursor LEFT into the submenu.

On a related note, menu layout should probably be mirrored (Toplevel menus
right justified.  Submenus appear left of the parent by default.) for
right-to-left written languages (Arabic, Hebrew, etc).  This should only be
done if the dialog layouts are flipped for the RL languages  (label-entry
for left-to-right languages, entry-left for right-to-left languages). 
Similar to what BeOS did/proposed.