I'm a huge admirer of Don Shank's art. Recently on his blog he posted some paintings he made using his iPad.
Above are a handful of paintings I did on my iPad with the Brushes app for iPad. I love this app. Yes, I've tried Sketchbook Pro, I like it fine, it's a great app, but I'm drawn more to the simple directness of Brushes. It's a pleasure to use. (Though there are a few things I wish it had (Saturation Slider please!))
Also, one of the coolest things is you can play back your painting right there on the iPad and watch as it re-creates itself. Even better you can email yourself the "actions" file for the painting to your computer and using the free Brushes Viewer application it can re-render the image up to 6x high and wide (that's 4608pixels by 6144 pixels!) The images here on my blog were shrunk down from that size to 2000 x 1500. It will export Quicktime movies too, I'll post some movies in the future.
If you like Shank's art, I recommend The Ancient Book of Sex and Science, which features Shank's work along with three other gifted animation artists: Nate Wragg, Scott Morse, and Lou Romano.