In case you haven't discovered it already, you might want to check out Artwork Tracker. Rather than being another art creation app (lots of those discussed in this thread already!), it's a submission tracking tool for keeping track of your artwork and submissions. It stores all the relevant info about your artwork (creation details, full-screen images, and so forth), client details, submission information, and statistics.
I released it last year, and just last week came out with an update addressing some artist suggestions. It also works on the iPhone and iPod touch, but is at its best on the iPad. I'm open to suggestions for feature improvements to make it even better