As indicated by the other answers, a USB 3.0 port alone is technically not sufficient to power the Raspberry Pi since the Rπ is incapable of negotiating a higher unit load over the default 150mA. However, in practice, many systems permit devices to draw much more than the specification maximum, and you may very well be able to power it directly off the port.
Before using this in any sort of "production" system though, I'd strongly recommend testing it extensively with all peripherals attached, etc. to make sure that the Rπ current draw never exceeds what your PC is willing to give it... otherwise you'll get funky behavior like you would using an insufficient dedicated power supply (random reboots, crashes, or even more bizarre behavior).
As an example, I have an old "portable" USB hard drive that's rated at 750mA, and has the twin-head mini USB cable (one for data, one for data+power)... however, I've had no problem running it on either my desktop or laptop plugged in with just the "data" connector, or even using a standard mini USB cable. For reference, both my desktop motherboard and my laptop are Asus... so I'm guessing they design with "non-compliant" devices in mind :)