Ha, I'm turning into an app reviewer these days, but I've found a favorite app for sketching called ASketch. It's the closest thing to drawing with charcoal I've seen in any digital format. I see from the iTunes reviews that the minimalist UI drives some people nuts, but I love it's simplicity--you just draw. I do highly recommend watching this video on the creator's website, as there were a few features I didn't discover on my own. Oh, and if anyone wants to buy me an iPad to play with I'll let you know how it works on a larger screen ;)
Blocking in the masses... then adding some detail.