"Actually, I just had another look at my manual and there is a fine print that says "Canon advises you to upgrade your gear to the newest Canon stuff as soon as possible. Canon will make sure that your new gear doesn't work with your old gear (see no radio addon for legacy 580ex flashes), but even if it does, Canon won't tell you and you shouldn't bother.""
This is exactly what I am afraid of...
Surely the pre-production version of 600EX-RT worked with 5d2 re: HSS. I hope Canon doesn't intentionally cripple
the production version of the 600EX-RT to force us to upgrade to 5d3 or 1dx.