Wait a second. Is this "yet another smart phone OS" or just Linux with phone apps? Or a Hypervisor?
It looks like the Ubuntu/Android was a hypervisor approach.
It looks like this phone approach is just ARM Ubuntu with phone apps. The presence and suggesting of using QML (Qt) for phone apps suggests this is no longer the hypervisor approach. it in fact looks like a replacement for the Nokia N9, complete with gestures.
So I don't think you're looking at another smart phone OS. I Think you're looking at the continuation of MeeGo (in a way) and the full-on, proper, Linux on on a smartphone (like the N9 was).
[Edit: to elaborate: You can write HTML5 apps, your can write QML apps. Your QML apps will work the same between phone mode, desktop linux, windows, OSX, (and soon iOS and Android) so only if you made a native Android or iOS app will you have any work to do. But the work you do will be able to be used on iOS or Android as the Qt support progresses on those platforms. ]
This is amazing in terms of potential in that Linux will get more QML apps, and your phone and your desktop will be one and the same... Also, we might get that multi-persona phone thanks to a modified login prompt.
I think this news is so big, we are having trouble appreciating how pivotal it is.