The question is in the title, but can anyone provide some details on what it is, how it works, and when it will be available?
It's not a "mobile version" of Ubuntu.
It's a special build of the Ubuntu OS that can be installed on Android dual-core phone, with this you can attach a HDMI monitor (and I think a bluetooth keyboard and mouse) and use your smartphone as a computer with Ubuntu.
As an extra, from that desktop you can access to phone data (like addressbook) and send / receive calls.
I think that this image clarify what is it:
Official page for more info: http://www.ubuntu.com/devices/android
It's similar to the Motorola devices webtop app that recent devices have.
You dock it and use your mouse and keyboard to use it as a desktop.
Coincidentally, Motorola has been using Ubuntu for a year on webtops.