There is no controller for USB 3.0 on the Patsburg PCH.
There is no USB 3.0 controller on the i7 oriented x79 branded PCH either and yet all of these motherboards, including Intel's, have USB 3.0 included.
http://www.xbitlabs.com/articles/cpu/display/lga-2011-idf_4.html
if Intel's reference board for the Xeon E5 (socket 2011 ) also had a USB 3.0 component in the design then whatever Apple tweaked to get to their design would also likely have it.
The primary reason for it to be missing would be Apple laziness and/or desire to kick the can down the road for the required software development required ( xHCI driver framework).
Microsoft has been working on native USB 3.0 support in Windows 8 for a long time.
http://blogs.msdn.com/b/b8/archive/2011/08/22/building-robust-usb-3-0-support.aspx . In 2012, USB 3.0 is going mainstream. So it is not surprising that the new PC designs being introduced at Q4 of 2011 are mostly on board with that trend.
It is not something Apple can just wake up some morning after Intel did all the work after weaving it into the core chipset. Getting the xHCI bugs worked out on the Mac Pro would make deployment to the rest of the Mac line up go smoother.
It is going to be pretty lame when the vast majority of the upcoming "ultrabooks" have USB 3.0 (even with the highly limited space/battery requirements) and the Mac Pro doesn't. Really lame.
The key source of how lame it is lies in the fact that the Xeon motherboards are on a two year design cycle. By not planning to add USB 3.0 at the end of 2011 would mean that it would be at best
late 2013 till the Mac Pro got USB 3.0. That's ridiculous in the context it is currently almost uniformly deployed on new > $1,000 computers that are not old design retreads. That would demonstrate that Apple has its head deeper planted in the sand with that kind of move. Or perhaps Apple is going to wait until Lion's successor ships to finally step out of the reality distortion field and acknowledge that USB 3.0 has deep traction.
For the other Mac designs.... sure there are board space constraints that hinder it being added until it is weaved into the core chipset. The Mac Pro doesn't have those limitations. They also have shorter motherboard design lifecycles. It is simplier to addd USB 3.0 than jury-rig some Thunderboard solution to a Mac Pro.