No reference; done in Autodesk SketchBook Pro for iPad with Wacom Bamboo Stylus (thanks Clover for the iPad + stylus).
Comparing to SketchBook Mobile on iPhone, which basic functions such as changing brush size and opacity are quite tricky to operate, the iPad version is so much more straightforward to use. Drawing with a stylus is easily more accurate than using fingertip, and feels more comfortable.
