I was looking at this image from the back of the NXT demo box, and I noticed something. In the upper right corner you can see the text "QuickStart Guide - in just 30 minutes, your first robot is in action."
I may be wrong, but I think that the robot shown there IS the robot that can be built in less than 30 minutes. This bot appears to be the equivalent to the RIS "Pathfinder" bot from the Constructopedia (version 1.5 is what I have). Both are wheeled bots and (from what I can see) neither has any sensors configured.
All NXT robot images (with wheels) I've seen so far appear to be variations of this bot. Other bots may be built in 30 minutes or less, but I think this is the one that LEGO may be using to demonstrate the quick build.
For this reason, I also think the QuickStart Guide has only 1 design (with maybe some suggestions for modifying it). The other information is probably in the 2nd book (with the CD on top of it in the image).
I bring this information up only because I think the other bots may be more time consuming - I don't think we can assume that all 18 Challenges with the kit can be built in such a short time.