The existing 24-70 is already quite ready to resolve a sensor like the 5DsR, IMO, so there's really zero incentive for Canon to make a mk3 in the next 2-4 years at a minimum. They've already made it very clear that IS is for their teles and f/4 zooms; if you want stabilization in a 2.8 zoom you should look to Tamron. They cost less than half as much, and are almost as sharp and almost as reliable to focus. Emphasis on almost, unfortunately. A general hobbyist shouldn't think twice about buying either the Tamron 24-70 or the 15-30, but a working pro should at least think twice about which 24-70 they want to make their "workhorse". Personally, as a full-time wedding shooter, I hate 24-70mm in general, and I dumped my 2.8 in favor of a 24-XXX f/4 a while ago. I'd rather have an ultra-wide zoom, a couple primes, and a tele zoom. But for walk-around stuff where you also "need" the aperture, I suppose the existing Canon 24-70 2.8 mk2 is a champion.
=Matt=