I am, for reasons of ergonomics, protecting the screen and providing the maximum available real estate to both screen and keypad, a fan of the flip design of phone. See most famously the impressive engineering but flat and unfinished styling of the Motorola Razer/RAZR and the Samsung A701 which has been pleasing and infuriating me in such successful turns that I'm beginning to think that it's a tool of psychological warfare.
But there's a problem with this position, which we shall place under the "right took for the job" file. The problem is this: GPS.
If you build something like GPS into a device, it works a little better if you make it so that the screen is visible all the time. Which is okay for a sliding form-factor, but tends to bugger up a flip. Which gives me somewhat of a situation flavoured faintly like a dilemma. The obvious solution is to do what flip-phone stalwarts Samsung and Motorola (or what remains of Motorola, anyway) have already done, which is to put a high-quality screen on the outside of the phone as well. But that sort of redundancy, of duplication of services, offends what's left of my sense of elegance of aesthetics and is no solution at all.
Neither is relying on voice commands, through a speaker or headset, when the phone is closed. So we move to a slightly less inelegant solution, the flip-twist. This design is hardly new, having been used at least since Sony was building a Palm-powered line of PDAs glorifying under the ludicrous name of Clie (pronounced Clee-ay, and with an accent I think), and having since defined the tablet PC, as well as appearing on several awesome models of Sharp's Zaurus Linux-powered PDAs and the odd (and sometimes extremely odd) mobile phone. The essence is that the device hinges open to reveal the screen and then, if desired, rotates through 180 degrees and folds back down over the keypad to leave you with a compact device with, crucially, an exposed screen.
Nokia did half of this design on a line of camera-focused phones that they appear to be letting quietly die (the final one being the N93i), and Samsung have recently introduced the same hinge into a phone focused on mobile TV and glorying in the defensive moniker of "Widescreen". Sad, I know. In fact, almost as sad as Samsung's website. Those devices used an offset rotating hinge so that the screen could either be used like a video camera's flip-out screen, with a fat camera mounted inside the usual hinge and pointing sideways, or like a portrait-mode screen for peering intently at tiny little videos where you can't see any of the detail or read subtitles.
So it can be done. Should it? good question. Will it? highly unlikely, I'm afraid. There are candybar phones with GPS, and sliders, so why would anyone bother? Mind you, if anyone does, it will almost certainly be Nokia who does it.