Excellent. SP2 has cured the 'VISTA hides the shortcut submenu' problem (Thread #1228167).

However, submenus open even when the originating bar is disabled with SKIP, as below. I now can't remember if this was the standard behaviour beforehand.

DEFINE POPUP _popShortcutMenu ;
		FROM MROW(),MCOL() ;
		MARGIN ;
		RELATIVE ;
		SHORTCUT
DEFINE POPUP _SubMenu ;
	SHORTCUT RELATIVE
DEFINE BAR 1 OF _popShortcutMenu PROMPT "Item 1" 
DEFINE BAR 2 OF _popShortcutMenu PROMPT "Item 2 With sub" SKIP
DEFINE BAR 3 OF _popShortcutMenu PROMPT "Item 3" 
ON BAR 2 OF _popShortcutMenu ACTIVATE Popup _SubMenu 
Define Bar 1 Of _SubMenu Prompt "SubMenu 1"
Define Bar 2 Of _SubMenu Prompt "SubMenu 2"
ACTIVATE POPUP _popShortcutMenu