It looks like the countdown to Android 4.3 for Sony Xperia devices can now begin. A new build version for the Xperia Z1 has been certified by the PTCRB website. Firmware version 14.2.A.0.290 made a brief appearance on the Xperia Z Ultra the other day, this is believed to be the Android 4.3 version for these two handsets. This new firmware has been certified for all versions of the Xperia Z1. We would hope to see it released in the next couple of weeks, especially with Christmas fast approaching.
Update: The same firmware build 14.2.A.0.290 has also been certified for the Xperia Z Ultra.
Thanks Deki and Rafael!