We've asked AT&T about the availability of the Dual Screen for customers on existing lines.

Suffice it to say that the phone becomes a tougher sell if you don't get that attachment for free, though. It's an upgrade over the regular G8 with a larger 6.4-inch OLED screen, a larger 4,000mAh battery and an updated interface, but it's not a fundamental shift in design or performance. If you're content with a single screen, some of the other more conventional designs might fit the bill.

Update 10/22 1:54PM ET: AT&T tells Engadget that the Dual Screen will be available by itself for $200 after the free promotion. That's not an exorbitant price, but you will have to think carefully about how much you want an extra display.