A small firmware update is now rolling out for the Sony Xperia Z1 in the form of build version 14.1.G.2.259. The update appears to be for branded devices only and has currently been released to those on the Telstra network in Australia. We doubt the update will land on generic devices, so everyone else will have to look forward to the Android 4.3 update in December.
Via XDA and Telstra Support.