Form factor means everything; look at how many new iPad only apps there are, strictly due to increased screen size. Jobs made this clear when he nixed the 7" iPad.
Is the Droid Pro a tablet? What about the rumored Playstation Phone? Tablet seems to imply a form factor, and I don't think that's what is important in how we categorize these devices.
It's like if we called PCs "beige boxes" when they came out or if we called TVs "two through thirteen dial machines".
I expect the form factors for these devices to continue to evolve and improve, yet I think the general category of device will be the same (much like how a 3D LED HDTV is still a TV, although looks very different than an old B&W TV from the 50s).