Where did you hear that?
I don't think there's been really any solid information at about the 5D3. You're more likely to hear specs about the new Stealth Bomber than the 5D3.
I would think that they would have to at least include 720 60p. There's been such a big request for that. With that said, Canon isn't good at listening to what customers want at all. Look at the Moire.... people have been screaming about that since the 5D2, and 4 cameras later, it's still not fixed.
But yeah... I would say slow motion is a BIG one, along with an improved Codec (4:2:2 hopefully) codec and no Moire = the 3 biggest requests.
That really is not a tall order at all for video, if Canon doesn't at least deliver that, for video, the 5D3 would be a disappointment.
As for using iMovie for Slo-Mo, it's definitely a ghetto solution. It rarely looks good doing slow motion in post. Using Twixtor only work in certain circumstances. We need real slow motion.