Finally bought several stylus pens designed for touch screens. What follows are samples created as I bounced around several drawing apps. You can get really nice fluid curves with these pens. Unfortunately, most of the iphone apps are saving at a resolution that doesn't capture all of the info that is there on the screen. The ipad will be a different story, as the resolution should go up to print-like resolution.
First up is a drawing started in Vellum, and then painted on in Brushes. Any fine drawing in any of the samples from this post was done using a stylus pen.
Next are two examples done in VELLUM attempting to show how nicely curves are rendered, despite the resolution. These look great on the iphone screen from within the app.
These last ones were done in Ink Dabbler, and again show the fidelity and accuracy you can get using the pen in certain ways. Not sure if the stylus pens work for gestural drawing, in the way that a fingertip might, but worth having.