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.
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.