The specs for my PC's motherboard don't mention any upper or lower limit for its built-in USB 3.0 ports.
What is the minimum current that any (standards compliant) built-in USB 3.0 port should be able to provide?
Is a USB 3.0 port suitable for powering my Raspberry Pi?
A unit load is 150mA for USB 3.0, and without further configuration, it will only supply 150mA. USB 3 ports, however, can supply up to 900mA (or 6 unit loads) by requesting higher power. The current schematic does not allow for this, and so, the only way to this is to insert hardware between the Raspberry Pi and the USB 3 port to request higher current.
You could only run the Pi directly from a USB port if it has been specified as a dedicated charging port
These ports can supply the required power, but have the data lines shorted so they are unable to transmit data.
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 :)
