At FOSSGIS I was asked to try to install qgis on a very small android phone, I think it was a 3.2″ screen. the install went smoothly after making some space but then the problems came because of the small screen.
Eventually I thought about setting a smaller font size to make the UI scale more, the problem was that it was impossible to get to the size setting because the UI was to big.
As a workaround I created a QGIS.conf file with this content
[General] IconSize=32
and pushed it to the device using the android debug bridge like this:
adb push myQGISConfigFile.conf /data/data/org.qgis.qgis/files/Settings/QuantumGIS/QGIS.conf
On the next start the whole gui was nice and small and fitted the screen.
Here some screenshots from my Samsung galaxy 9000 with 4″ screen and a video demonstrating digitising (with pen and fingers), GPS, compass and zooming on the phone.

This video shows QGIS on a Samsung I9000 Galaxy S Android smartphone with 4.0″ screen. the point size in settings->option->general is set to 4