Apparently iPhone is not a perfect drawing tool, but if you want to express your drawing inspiration at any time, the paint app ArtRage for iPhone should be a nice solution.
ArtRage for iPhone is a powerful physical paint simulation developed by Ambient Design, and designed for iPhone and iPod touch. The iPhone app has a simple and intuitive user interface, and comes with 7 drawing tools including pencil, ink pen, crayon, oil brush, palette knife, paint tube, and paint bucket, while each tool is adjustable plus powerful color palette and eraser in order that you can create awesome art works on your iPhone or iPod touch, moreover, professional layer function allows you conveniently modify your painting. Apart from that, the draw app comes with 8 different canvas, and you can import your photo as tracing image for easy painting.
ArtRage for iPhone with a size of 8.0 MB is compatible with iPhone, iPod touch and iPad running iOS 4.3 or later. The iPhone app is free for a limited time at present. If you’re interested, jump to App Store for more details.