Mac Life has put together an interesting article on why we probably won't see USB 3.0 natively on the Mac. It's worth a read, but the long and short of it is this: Light Peak -- the ultra fast 10 Gbps, one size fits all (USB, Ethernet, FireWire, SATA and PCI Express) optical cable replacement that's been dangled before our eyes, but not made a reality, yet.
Light Peak was first demonstrated by Intel back in 2009 on a hackintosh running Mac OS X -- which ruffled a few feathers. At the time, Engadget reported that Apple was behind the technology, with discussions between the two companies developing the technology dating back to 2007, and that (according to an extremely reliable source) Light Peak would be introduced to the Mac line-up in the autumn of 2010 with iPhone implementation in 2011.
Since then, we've seen USB 3.0 come to market (to a small extent, and not natively on the Mac), but there's been no sign of Light Peak. Unfortunately, at the 2010 Intel Developer Forum, Electronista reported that the chip-maker said it didn't expect Light Peak to come to market until 2012 -- making its original "...maybe not next week, but soon" comment a little overly optimistic.
Regardless, and getting back to the Mac Life article, it's unlikely that we'll see Apple natively support USB 3.0 because Light Peak is on the horizon -- be that a distant one. In my humble opinion, with the slow adoption of USB 3.0, and its imminent obsoleteness with Light Peak around the corner, it wouldn't make sense for Apple to introduce a technology that's about to be replaced by a better one. As paradoxical as that might sound, Light Peak will not be an upgrade, it will change the way we connect peripherals to our computers, which is likely why it's worth the wait.