Let's say its 2lbs or 2.5lbs and less than .5 inch thick. Big foot print, tiny everything else.
1. Would something that light be ultraportable?
I think so, but take your point
2. Would there be a market for it?
Yes. I will buy immediately and in my personal case would prefer to smaller screens.
I like a bigger screen and non-cramped keyboard. I own a 10 and 12 inch (widescreen) ultraportable from Apple competitors and find the screen hard to watch and in the case of the widescreen almost no vertical viewing area (problematic for word, powerpoint and some web surfing.
I also have an old 12inch PB and that screen is fine (the 13.3 letterbox has the same vertical viewing area). I like it and use it every day.
The trouble with the form factor you describe is:
1. It's still too close to the current MacBook. Yes, it's much thinner and lighter, but it's not the sort of device you can just throw in a small bag or even a large coat pocket. You'd still need a dedicated way to carry the thing around, and depending on how much case integrity you sacrifice for the thinner form factor, you may need an
even more substantial bag than current laptops use to prevent it from being damaged.
2. Even if you're correct and there is a market, the market is not nearly as large as that of a "true" ultra portable. Why would Apple put the R&D into a smaller laptop and then release something that isn't far enough from their current offerings and wouldn't capitalize on what the public is clamoring for? You can say what you like about what form factor you prefer, but I think it's clear to even a casual observer that Apple users (diehard and casual alike) are simply
dying for a Mac you can hold in the palm of your hand, or at the
very least a replacement for the old 12" PowerBook form factor you mention. This looks to be neither of those.
What is really needed is something to bridge the gap between the iPhone/iPod Touch and the MacBook.
Something with a 7" to 10" display, 32GB or more of storage, a decent hard drive and standard ports. Price this at $1,000 to $2,000 depending on the configuration and you'll have yourself one hot seller...