I expect it will be whatever latest 24-105 or 24-70... but I find that an uninspired choice...
It seems the first buyers/early adopters of the 5D M3 will be those upgrading from a 5D M2. And there may be some like me who decided to wait on a full-body due to the rumors and go for the 7D for its albeit crop excellent action shoot credentials.
For those with 5D M2's, I suspect most already have one or perhaps both of those workhorse lens. Why do they need to buy the kit?
I am in the 7D camp. I fully plan to buy a full body, but the expectations of a M3 convinced me to go the other direction - and with the $200 rebate last spring, the 7D was a good interim step for building up a full system. If there are any retailers out there doing their own kits - I hope that look at offering kits that replace EF-S situations of the crop body world.
My own personal wish kit would be a 5D M3 with 16-35mm f2.8 lens. I need to match or replace my current 10-22mm EF-S and a 16-35mm EF would be perfect. I already got the 24-105mm, so like the current 5D crowd a 5D M3 with that kit lens would be a big yawn. Hell, stuff a 50mm f1.4 as someone suggested! or maybe the 135mm f2.0 (test driving this coming weekend at an event).
Of course if this waiting game goes on too long, a 1Dx may become part of my budget.
Get creative Canon! - or at least take the cue big retailers. The 24-70 and 24-105 seems like a loser for the early adopters.