I read up on the Ubuntu for Android thing and I'm confused, Specifically on these topics:
When you're away from your home computer what OS does your phone use?
Is the OS literally only for when you connect the phone to the PC?
The phone will always be running Android as its base OS. When you dock it (the dock being plugged to a large screen, a keyboard and a mouse) the phone fires up a X session running Unity. When you remove the phone from the dock the X session closes and you are back to the Android touch interface.
Although this has not been discussed in the various videos released, there is no reference to a special behaviour when connecting the phone to a PC. Thus I would expect it to behave as usual, and simply mount the phone SD through MTP.