Debian bug http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=483702 seems to address this, albeit in an indirect manner; apparently, the manual cannot be shipped with Virtualbox in Debian for licensing reasons. Presumably, Ubuntu could and probably should include the manual, since Ubuntu has more relaxed criteria for what can be included.
Upstream bugs http://www.virtualbox.org/ticket/2364 (manual) and http://www.virtualbox.org/ticket/2365 (web site) request virtualbox.org to relicense their documentation so that it could be included in Debian.
If the documentation cannot be shipped with the virtualbox-ose package, perhaps the Help > Contents menu item should simply be disabled. There's still the link to the Virtualbox web site in the Help menu.
More tangentially, http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=440609 notes that there are also no manual pages.