USB3 support is native on Ivy Bridge. It wasn't on Sandy Bridge, and required a optional controller. There is literally now no reason for Apple not to offer it on their new iMac unless they are ordering custom-gimped IB CPUs from Intel, which would probably cost them more and would be a completely insane move.
Its got nothing to with the gimping the CPU or custom CPUs since its not the CPU that decides whether you have USB 3 or not (or whatever ports for that matter).
USB support is built into the chipset, which is basically a motherboard controller that controls the communications between the CPU and external devices.
Intel has announced 3 chipsets for Ivy Bridge. All 3 support USB 3 natively (4 USB 3 ports and 14 USB 2 ports). But 3 of last years Sandy Bridge chipsets can also utilise Ivy Bridge CPUs (Z68, P67 and H67), which, as has been pointed out, do not support USB 3 natively and motherboards require an extra optional controller.
In 2011 Apple used the best and most expensive chipset available for Sandy Bridge , the Z68. In fact, Intel was not even officially shipping the Z68 for a week after the 2011 iMacs were released.
So, Apple could choose to use the Z68 chipset from last year and save themselves some cash by reusing last year's motherboard design. But this will mean that we get Ivy Bridge CPUs without USB 3. Lets wait and see.