A quick mention to a couple of mobile apps that have impressed me. Live Sketch (iPhone, iPod Touch, and iPad) and Live Sketch HD (iPad) by Yan Rabovik seem to be great apps to make good looking sketches with your iPhone. Simply, draw fast for thin lines and draw slow to make thicker ones.
The drawings look like pencil on paper. Artwork becomes effective and bold; but, in my opinion, the app effects seem a little too heavy and make drawings look similar in style, diminishing the artist's signature.
Some drawings by users can be checked out in Rabovik's twitter account .
Overall, the app seems worth giving a try.
($1.99 ea. on iTunes store)