The iBook nano (wish)
Let me dream. :) The keyboard of the iBook nano can be a little bit smaller than "real size". 80 or 90 percent, I'd say. That defines the width of the device. The trackpad doesn't have to be too big either to save space, too. The screen should be wide, of course. An optical drive is not necessary. (You can buy an external USB-2 DVD-RW, Apple could provide one, too.) The "harddrive" would be 32 or 48 GB NAND flash memory. Its processor would be of the ultra-low-voltage variant, intel has been doing those for a while, they're actually good.
If you connect the device to your "real" Mac, it could either act as the main harddrive of the "bigger" Mac, act as your home folder on any Mac or simply synch (probably better, because that'd mean a good backup, too) to that Mac. It'd make a very good ultra-portable with no moving parts besides the keys. It could give you six or more hours of battery life and actually be _the_ device to watch movies on.
The real problem would probably be its price. But one can dream. (I'll continue to repeat this until it happens, I guess...)