A Nanobook Dream
What I want is a functional laptop, one that runs my usual software, small enough so that I can conveniently carry it with me most of the time as I did with my Psions. Fitting in a usable keyboard and screen probably requires the largest form factor that can be conveniently carried. Experimenting with a piece of cardboard, that appears to be about 4.125"x7.5". If reasonably thin and light, that goes comfortably into the inside pocket of a sports jacket. That would require me to wear a sports jacket or equivalent--or else some sort of belt case--but then, I started wearing sports jackets routinely when I started carrying my first Psion. It would fit into a pants pocket, but I'm not sure that's a safe location for an expensive and somewhat fragile item.
For screen and keyboard the important constraint is physical size. The keyboard of my desktop is 11", so 7.5" allows about a 2/3 size keyboard, which I think would be fine once one got used to it; the Psion Revo managed to fit a just barely typeable keyboard into less than 6", although I have always suspected them of using magic to do it. My Nokia 9300 smartphone has a 640x200 screen that's about 4" x 1.25", so 1000x600 should fit comfortably on my nanobook's screen.
I don't know enough about the internals of pda's or laptops to be confident how much space the rest of the machine would require. But my guess, from things like the iphone, is that it could be managed with a thickness of no more than an inch and a weight of well under a pound. Even that much weight might be a problem; my Psions managed to do minor damage to a lot of sports jackets over the years. But that's a price I was, and am, willing to pay.
My one additional suggestion is that the machine might include a hard drive--very tiny hard drives now exist, and I think they still provide considerably less expensive mass storage than flash memory. The machine could have the operating system in flash memory to permit almost instant booting, like my old Psions. The hard drive, if there was one, would provide additional storage, and would remain off, drawing no power, unless needed.
Hitachi is supposed to be bringing out a 20 gigabyte microdrive this year. Add another 20 gigabytes of flash, a gigabyte or so of RAM and the lowest power version of the cpu used in current Mac laptops and you have a functional laptop capable of running the current MacOS and applications. Small enough to (just barely) fit in your pocket
I want it.