After updating to iOS 6, iPhone will not charge from Mac Pro USB ports
Solution 1:
This is a rather peculiar problem, and I have not found any more widespread reports of this problem.
Try rebooting everything; that often helps.
Solution 2:
I'm not seeing anything like this with my iPhone 4 after updating to iOS6.
So I'd expect the usual suspects; try restarting both the Mac Pro and the iPhone.