The FAQ is clear that you won't be able to save any games on USB device, but sin't clear about whether USB devices are supported in any form (from a quick glance. Haven't read it all the way through).
Actually it says you can use a USB drive to update the firmware so they are definately supported at some level.
Will I be able to install system software update 1.50 via a USB drive?
Yes, you will be able to download the firmware to your PC and transfer it to PS4 using a USB memory stick.