and having used linux on an early tablet, I can attest that its not fun running most linux applications with touch on an 8" screen (with a pen in my case). Running Ubuntu on a phone would be pretty useless with current linux applications.
I think that Ubuntu is realizing that it won't be long before we have enough power in our pockets to run desktop applications, using the appropriate desktop peripherals. We aren't there yet, but I think it is an inevitable result of convergence.
When it does happen, Ubuntu will be ready, and perhaps have an advantage over other players by getting into the game now. Ubuntu will either have to learn to live with one of the existing phone app infrastructures for use in phone/teblet mode, or encourage development of apps appropriate to a phone or tablet.