Maybe I'm missing something obvious, but I don't see the submenu widget anywhere.
I have one menu.
I've set the widget to use that menu.
I've confirmed that a text widget in the same sidebar appears just fine.
Menu: Primary (twenty ten theme location, also tried unsetting this)
Start Level: tried 0 and 1
Show submenus: tried both
I resaved both my menu and theme location.
WP_Debug is set to true and I see no errors.
I'm using a very simply Twenty Ten child theme.
WordPress Importer is the only active plugin.