I have tried squeak on my android devices, it basically runs on android yes but it does not behave well with multi touch its practically unusable. Basically it does not understand when someone lifts his finger from the screen , so it can get clicks but its sticky clicks.So moving windows around and clicking on things is a pain and not usable at all. I suspect something similar applies for pharo too.
If you want to develop for mobile devices your best bet right now is Amber. Amber tries to be as compatible as possible to pharo and compiles to javascript. You should be able to use pharo as a backend if you want to but I have not tried to really confirm this, I dont see any reason why this wont work. The advantage of amber is that any app you develop with it wont be just a mobile app but also a web app. Using pharo for desktop apps is of course perfectly doable and recommended or as a backend to web apps. But if you have a tablet and want to focus on this I would say that Amber + Pharo should seal the deal for you and you will give you access to loads of stuff you can do because amber will give you immediate access to all Javascript libraries and that can only be a good thing :) Also javascript is probably a very good way to develop Android apps that also work on iOS.
