Oqo's 01+ measures just 124x86x23mm (width x depth x height) yet runs a full version of Windows.
Hardware specs are impressive, with a 30GB hard drive, 512MB RAM, 800x480 5in display, Qwerty keyboard and track point, Wi-Fi, Bluetooth, USB 2.0 and FireWire. The only noticeable deficiency is the 1GHz Transmeta processor (remember them?).
The latest version includes a touchscreen and is compatible with Microsoft's Tablet PC OS. Prices are a little steep at around $2,000 (about £1,130).
More info available here.