Only North America has this issue with carriers
The rest of the world does not suffer the carrier-muscle lock-in of the N.A. market. You can buy the phone you want, and buy the SIM & contract you want, and there are plenty of independent retailers who will do a competitive bundle so you don't pay for the phone up-front. Ubuntu may well do better outside the USA (Canonical is a London-based company).
I think the big issue is apps. It takes time to build up a wide enough range of apps before heavy users of one patform will migrate to another. If Ubuntu could run an Android emulator, that would help pull Android users over.
It's why original Windows succeeded (you could still run DOS apps), and why Palm's WebOS didn't (you couldn't run legacy PalmOS apps). You have to ease transition.
