When I checked out the iPad last Saturday, I tried typing. I did not spend enough time to really try touch typing, but it is hard imagine being able to do so without the tactile feedback of a real keyboard. For me the little bumps on the F and J keys of most keyboards are important for orienting my fingers over the home row, which is the foundation for touch typing. Without those bumps on the keyboard my fingers sometime float lower or higher than where they should be, resulting in my typing gibberish.
Still, I could see in my short trial that I might be able to type at a good rate while looking at the screen because of the placement and size of the buttons of the on-screen keyboard. Today I learned about TapTyping, which is a $5.99 iPad app that provides typing drills that help you to improve your typing skills. I plan to buy this program once I get my iPad so that I can type as quickly and efficiently as possible.