phranque - 10:00 pm on Sep 23, 2012 (gmt 0)

That's an impossibility to do with http://example.com/ simply because it doesn't exist.

at the bare minimum the virtual server at your example.com hostname should "exist enough" to accept a HTTP Request and respond with a sitewide 301 status code redirect to www.example.com which is your canonical hostname.

ideally the server hosting at example.com should be smart enough to redirect only to 200 OK results on www.example.com by avoid redirects to error pages or redirects to additional redirects.