I'm glad I found this thread. I am currently taking on the same project and have gone through a very similar thought process. My current road block was finding an open frame touch screen. I'm surprised that there aren't more on the market. Two years from now I'm sure they will be very common.
bleego, thanks for the link to the screen you are considering. Their blog post describing their delays is entertaining. Generally I don't like a company that takes your money and doesn't deliver product but they seem to be honest and upfront about the development process.
Update on 7inch status | Chalkboard Electronics