A little experiment done with the demo of Colors, which doesn’t allow saving work… hence the screenshot with the toolbar showing.
One of the most irritating things about sketching on an iPod is no pressure sensitivity, so no way to make the brushes get larger or smaller or more opaque or more transparent simply by how hard you press on the screen. Colors gets around that by using the accelerometer and changing the brush size or opacity depending on which way the iPod is tilted. It’s a pretty cool work-around, though it takes some getting used to (‘let me tip this a little so i can see it from a better angle WHY IS MY BRUSH SUDDENLY TINY oh. yeah.’)