Way back in December 2011, Sony began releasing 'alpha' developer ROMs for some of its phones being upgraded to Android 4.0. Then it released beta ROMs that did slightly more stuff. Now it's done the same with Android 4.1 for the Xperia T.
These ROMs are developer-facing in every sense of the word, though, and aren't intended as a way for power users to get early access to the next version of Android. Most things that consumers, you know, use, are turned off.
- Google Mobile Services (GMS) apps are not included (for example, Gmail and Google Maps).
- Modem is turned off and therefore the SIM card will not be recognised (this also means you can’t make any phone calls).
- Bluetooth™ is turned off.
- Wi-Fi® is turned off.
- FM-radio is turned off.
- NFC is turned off.
- UI look and feel is not updated to final Sony Jelly Bean software.
The point of these ROMs is more to get builds in the hands of app developers and the development community at large. There's even an official XDA thread made by Sony regarding the release.
To flash the build, you'll need to be using an Xperia T with an unlocked bootloader. Check out the Sony Mobile Developer World blog post for more.