There must be something weird going on with manufacturers lately - first Huawei released an alpha build of ICS for the Honor, and now Sony Ericsson (soon to be just Sony) has released an ICS ROM for certain unlocked Xperia devices, including the Arc S, Neo V, and Ray.
Since it's an alpha ROM, it's not made for day-to-day use, as it is limited to just a few functions at the moment. As far as things that aren't working are concerned, here is the list, as provided by SE in the official blog post:
Google Mobile Services (GMS) apps are not included (for example, Gmail, Google Maps, etc.).
Sony Ericsson has taken a lot of flak in the past for its slow roll-out of Android upgrades, however this no longer appears to be the case as today they have announced that alongside the launch of their newest Android device, the Xperia neo V, all 2011 Xperia smartphones (i.e. the Xperia arc, Xperia PLAY, and Xperia neo) will be upgraded to Android 2.3.4.
The update will bring a number of new features to the Xperia phones including video chat support for Google Talk, easier Facebook integration with "Facebook inside Xperia", swipe text input, screen capture support, and the ability to connect USB peripherals through the Sony Ericsson LiveDock.