It seem to me that it will be a difficult task to achieve significant market penetration.  You already have very large, resource rich companies involved in mobile OS, and I doubt they are going to quietly allow another competitor to come along and try to take a piece of the pie.  Apple is obviously not going to have anything to do with Ubuntu since they have their own OS, so there goes much of the high end market.  IT departments are unlikely to embrace another OS for employee use just as a practical matter.  This is not to say that there will be no adopters, especially if it is stable and easy to use, as jimlynch noted.  Tech savvy consumers that are already familiar with Ubuntu are one potential base, as are the iconoclasts out there who enjoy adopting things that are outside of the mainstream.  I would not be surprised to see a disproportionate rate of Ubuntu adoption among IT professionals, mono-skiers and Moto Guzzi motorcycle riders. :)