Though periodic rumors keep us guessing, I just don't see 100-400L update coming anytime soon, for reasons similar to the ones I listed for the 14-24L earlier, granted only point #2 really holds for the tele: http://www.canonrumors.com/forum/index.php?topic=12297.0 Canon simply may not see the release of an updated 100-400L II in its best interest right now.
The current 100-400L is a good zoom; they still sell a ton of them; and increasing its IQ any more could potentially cut into other new entries and higher priced lenses... including their yet to arrive in real life "200-400L + 1.4Ex". They would want you to upgrade into the higher price rung if you are that eager and willing.
Remember, Canon has left a lot of lenses from early years as they are... this zoom is rather new in that context.
I have given up, though I naively thought that it will be coming and I can replace the push-pull, and may be get more complete/full weather sealing.
A rumor here hailed earlier: "2013: year of the 400 lenses?" And I say, only if you are 420-friendly.
If you want a 100-400L get it now, no point waiting. I don't see such a lens showing up, regardless of the rumors that keep us on our toes.