Starting off the update rollout will be the Xperia Arc S, Xperia Neo V and Xperia Ray, and the update should start rolling out in mid-April for those devices. Then in the "late May/early June" window, Sony will be the Xperia Arc, Play, Neo, Mini, Mini Pro, Pro, Active, and the Sony Ericsson Live with Walkman as well. If Sony can pull it off, that would be one of the best 2 month stretches we've seen in the Android ecosystem.
Unfortunately, none of these updates will be OTA, and will require a tethered update. Apparently, this is because Sony wants users to make a "conscious and informed decision to upgrade." That's a somewhat strange way to phrase it, and will most certainly make for a slow rollout even if the updates are made easily available, but we'll have to see how it works out.