The Sony Xperia S has been kicking around in the Android Open Source Project for a while now. It never was an official target, but with the work from Sony and JBQ it was a usable option for those willing to tinker around, and by most accounts a success. With things ramping up and preparing for the AOSP to move on to Android 4.2, it has been removed.
The project will still live on, though, as Sony has set up a dedicated git for the Xperia S at their GitHub space. Interested parties can still hack away at Android 4.1 for the device, with plenty of community and official support. Full instructions for pulling and building are available, and folks interested should find the experience hasn't changed much.
As for why it was removed, Sony engineers explain that Google can't commit time and resources to maintaining anything other than Nexus devices in the official AOSP. We still see the Pandaboard in AOSP though, so there's more to the issue. It appears that Sony is unwilling to release all the binary files needed to boot the Xperia S into a usable state, which is a good reason to have it removed from the AOSP. We're not sure if the move was mandated or voluntary, but it certainly wasn't unexpected with needed files being withheld.
Update: Jean-Baptiste Quéru, Google's "Android Open Source Tech Dude" and wrangler of the huge AOSP, has taken the time to reiterate that there is no conspiracy going on, and that Sony is now the company with the expertise needed to carry on the project, not Google. In addition, none of the current devices have all the proprietary files available, and never have. Thanks for taking the time, JBQ!
Sony, we applaud you for taking an interest in the AOSP, and for the time you've spent working with the community and Google to come this far. We're not so happy about the binaries not being made available, but it's good the see a strong base being continued by your own engineering team instead of just abandoned.