The attachable tablet keyboards can't really be used comfortably on the go - commuting, sofa surfing etc. A laptop has a sturdy, built in lap rest that can be used in just about any position.
IMHO, the killer product is somewhere between an iPad and Macbook Air - with an attached reversible clamshell keyboard, or detachable/attachable keyboard that can be used to support the screen at any angle without being on a desk (ie not some flimsy magnetic lock), so the device can be used like a tablet or laptop and used in either mode wherever.