What will this Microsoft device be called? Is it a phone?
Surface Mobile is the front runner, by far. 'Surface Phone' would seem inappropriate given that (voice) telephony would be a relatively minor feature on such a device. As with the other Windows 10 on ARM devices announced so far, this would have LTE baked into its core, though voice calls would need to be taken via a Bluetooth or USB Type C headset. What other possibles... 'Surface Note', anyone?
What OS branch will it run?
It will almost certainly appear with Windows 10 S, with the usual 'upgrade to full Windows 10 for free within a certain time period' qualifier. In terms of UI and the use of (Store only) UWP applications it will all be very familiar to current users of Windows 10 Mobile though. It will launch with whatever 'Redstone 4' is officially called.
Will it be an easy upgrade from a current Windows 10 Mobile handset?
Yes, fairly easy. Just sign in with your Microsoft account and all of your information, favourites and UWP applications will be present from the Store. You'll need to set up your Start screen again, of course, plus old Windows Phone (8.1) applications won't work.
What about internals?
It will be powered by a Snapdragon 835 or 845, almost certainly with 8GB RAM. Storage should start at 128GB and it's unlikely that there will be any expansion via microSD. Each AMOLED display section will be around 1080p, though at custom resolution because of the curved sections on one edge, around the hinge. Overall display resolution, when unfolded, will be around 2000 x 2000 pixels.
What forms can it exist in?
As shown in some of these renders, and as detailed in the patent drawings, the Surface Mobile can be carried fully closed and with the screens protected. In this mode, the curved screen edges provide status information and notifications, including an incoming phone call (taken via wired or wireless headset etc.)
Partially unfolding the device gives a 'laptop' like mode, with virtual QWERTY keyboard on one display. Folding further to 'all flat', the Surface Mobile becomes a 8" tablet, albeit with curved (though lit) 'gutter' down the middle.
Folding further back, beyond the 180°, puts the device into a tented mode, propped up for media watching, while folding right round on the double hinge to the full 360° gives a traditional(ish) smartphone form factor with a 6" display.
How do the two halves of the device communicate?
Through wiring in the double hinge and this will be a slight reliability worry, although Microsoft's testing procedures should ensure that the cabling is flexible enough.
Where is the battery, the speaker(s)?
Under one display (the half with the USB Type C port) is the battery, under the other is the motherboard and chipsets. The aperture on the side, shown below, could well be a speaker port. It's not clear what the designer/renderer had in mind here. Some rumours pointed to a generic 'Surface port', so it could be that. Any suggestions? Do remember that all of this, both renders and text, is just educated guesswork!
Where is the camera?
Almost certainly underneath the display glass of the 'right hand' half. That way it would be used for Skype video calls when unfolded and as a main camera when folded, with the left display acting as the viewfinder.
Does it need a power button?
It's tempting to say that a double tap on a display or merely opening the device up from the closed position would turn the display on, but in practice you can imagine the user frustration if this didn't work reliably, plus what about turning the display off without being forced to change the hinge position or wait for a timeout. Therefore, as rendered, a standard phone-like power button will be present.
Those volume buttons look awfully 'phone-like'...
They do indeed. In this render, at least. If this is accurate then maybe the folded Surface Mobile could be used as a traditional phone after all. I'd bet against it, mind you. After all, how many actual voice calls do you take or make on your high spec smartphone? It's about two a day for me, and that could equally well be farmed out to speakerphone or headset use.
Will a Surface Pen be included?
No, even though it's shown in the renders. The Surface Mobile will be Surface Pen-compatible though.
When will this be announced and at what price?
Everything now points to March or April 2018, with availability in the summer, and I'd estimate a price tag of around $1000 (£900?) This won't be cheap - the materials, specs and engineering effort all ensure a premium price tag.
What does all this mean for Windows 10 Mobile?
Ultimately, it means that this will likely be available before support/patches for even the oldest current Windows 10 Mobile branches (think Anniversary Update) cease. So, in principle, you can gracefully retire your Lumia 930 or 1520 or even 950 XL, and simply move to this slightly larger but much more flexible solution.
However, in reality, such a transformer device as a Surface Mobile will be compromised in some ways. The existing Lumia 950 range will remain top dog for imaging, the existing Alcatel IDOLs will remain top dog for media consumption, and so on. So you might as well eke out every remaining month/year of life in your existing investment. Not least because once the Surface Mobile (or whatever it's to be called) is available there will be a raft of cheaper clones and then you'll start to have more choice about the future within the Windows mobile world, about life after Windows 10 Mobile...