I had an idea on the day that the iPad was announced (yesterday, I think). Imagine 2 iPads, next to each other, touching, longer side to longer side, facing the same way. Now, imagine that they are connected by a hinge, and that they could close together all the way, such that the screens face each other, similar to how a laptop closes screen-to-keyboard. Finally, imagine that these two devices are the same device, rather than two separate devices tacked together. I think that this device - which I will tentatively call the iPad DS - would be good. It's like a cross among an iPad, a conventional laptop (or perhaps a tablet PC - I don't know much about them), and a Nintendo DS.
This layout has some advantages over the other devices:
-The maximum potential screen area is double that of an iPad.
-With the right settings, one vertical, landscape screen could act like a normal laptop screen, and the other screen could function as a keyboard. Thus, it could effectively be a laptop with a touch-screen, but it adds versatility, since it doesn't have to be.
-The ability to close it means that the screens are better-protected than that of the iPad.
-It would potentially be very nice for reading newspapers or books. Think of holding a laptop book-wise, with the keyboard replaced by another screen.
I estimate the cost for the basic version of such a device at $1000 if it's made by Apple. The cost of $500 per iPad is a good guideline, and there is added complexity, but at the same time, the hardest part of development is already finished.
I have no plans to buy an iPad, for many reasons. I don't have a job yet, I already have a computer that's new as of 3 months ago, and I like using my computer on my desk (keyboard down and monitor up). However, if a double-iPad were released, then I would seriously consider buying it once I have consistent work.