A strange, yet intriguing, concept. Hopefully they do a little more than just transfer the text to the screen-- the interactive capabilities of the DS is really a large part of its charm. With the stylus and touchpad technology, it would be possible to allow the reader to be a bit more "active" in the story.
Thinking about it, the "ideal" book to put on a DS or other handheld would seem to be the Choose Your Own Adventure books. I loved those books as a kid, but the DS format would allow more seemless adventuring and also potentially some great graphics and mini-games to further immerse the reader.
Still, intriguing. We do check out DS games at the library, so I think we'll have to purchase a few of these when they come out to see how well they work and how much demand there is for them.