Update: It looks like T-Mobile got cold feet. The support page has been reverted to its previous state, showing only the Android 4.2 update from November. Sorry, folks.
Do you use a Sony Xperia Z? Did you buy it from T-Mobile? Then check that Settings menu - according to this T-Mo support page, you're getting a taste of Jelly Bean 4.3 starting today. Of course these things tend to go out in waves, so those without patience can follow the links on T-Mobile's site and manually download and flash the new software (10.4.C.0.797) using the Sony PC Companion software.
The last time T-Mobile's carrier-customized version of the Z got an update was way back in November, when it was upgraded to Android 4.2. The vanilla international version of the Xperia Z got its 4.3 update in December, so this is a pretty typical delay for an American carrier's update. According to the support page, users can expect UI improvements to the UXP Air and WiFi calling apps, the SwiftKey keyboard replaces a similar one from Nuance, and a 5-bar signal is present instead of a 4-bar version.
T-Mobile's software update weighs in a 308MB, which isn't bad as these things go. It's recommended that you have your battery charged to at least 50% before beginning the update. Have fun. Oh, and don't ask how long it will take T-Mobile to get Android 4.4 to your phones - Sony has confirmed that the Xperia Z will get KitKat, but there's no timeframe in place.
Source: T-Mobile Support