A new firmware update has been certified by the PTCRB website for both the Sony Xperia Z (C6603) and Xperia Tablet Z (SGP321). Firmware build version 10.3.1.A.2.74 is only a minor update over the previous build (10.3.1.A.2.67) that rolled out recently. We would expect the same update to land on the Xperia ZL and Xperia ZR, but it most likely has not been certified at this stage. We’ll let you know when it gets released.
Update: Judging by the comments, this update is already rolling out! See confirmation below (Thanks xPnoyStar!)
Thanks Ben and Piran!