A four pounds 12" Mac... Isn't that called a MacBook? Ok the MacBook is 13" but still.
I want something REALLY small but that's still a real Mac. Give me a Nintendo DS Lite-sized ultraportable Mac with a 1.5GHz Intel Core Solo (or something similar that's very light on power requirements), GMA 900 integrated graphics (very low-end model), 512MB built-in (perhaps no extra memory slot either), 800x480 display and no optical drive. But it has to run OS X, not some watered-down, "ultraportable-only" operating system. Add one of those new hybrid flash/HD drive for good measure and a SD card reader built-in for real on-the-go usability for photographers. All those features would make for a real "PDA-killer".