Back in August of '12, Sony teamed up with Google to make the Xperia S an officially supported AOSP device. The project initially got off to a decent start, but after an issue with some proprietary software binaries that couldn't be released by either Sony or Google, the project was canned on the official side and moved to Sony's GitHub, where it can still be found today.
Now, the company is doing something similar with the Xperia Z, minus Google's interaction from the get-go. It has released full AOSP for the Z to its GitHub, along with most of the proprietary binaries needed to get things going. Unfortunately, both the modem and camera require binaries that can't be publicly released.
Still, the project is already off to a promising start – it boots, and the SD card slot, Wi-Fi, Bluetooth, GPS, LED, and most sensors are working. With that said, it's not meant to be a daily driver at this point, but more of a fun tweaking project for those in the development community who are interested in getting a pure Android experience on this device.
The code and all available binaries can be found on Sony's GitHub, and developers are encouraged to make themselves familiar with the code, take a look at the list of issues, and start tweaking. More information can be found at Sony's developer blog below.