How do you know? (And does it have a slideout keyboard? )

It is just logical deduction. If you update a device with a newer version without adding any major new functionality, but DO increase the introduction price by a large amount than you will sell less units than you did with the previous model on introduction. (a higher price results in a smaller audience).
Nokia wants to increase the audience for the internet tablets, so that doesn't seem right.
So it is logical to conclude that the higher priced device will have a major new functionality built in. Since the software for navigation is already available as an optional add-on for the current N800, I suspect it will probably be just that (easiest to implement).

But other things are also possible. Nokia could also have added a high quality video / photo camera or a 3G phone/data module or have added a lot of build in storage space (a hard disk or lots of flash memory, and hence adding video and audio playback features)

"Small" changes like a faster processor or TV-out or a slide out keyboard do not add to the usefulness of the device directly, so it is highly questionable if consumers will be willing to pay 100 euros extra for that.

Well, I would argue adding a slide out keyboard will probably increase the production cost fairly significantly too, similar to adding a GPS chip.

That could be true, but I don't think the production costs are the main factor in the price. Nokia Internet tablets are expensive when compared to similar hardware from other manufacturers. For example the ipod Touch or the Archos player.
Also the fact that the 770 and N800 decrease in price very strongly after 9-12 months indicates that nokia prices the devices according to their own strategies. There are NO other devices with this combination of features: Open source software and hardware functionality. The IT's are unique, so Nokia can price the devices as they see fit (depending on the audience Nokia wants to target).

You missed my point.

How do you know that the price is 100 Euro higher? Where are you getting the concrete information that this is true?

huh. If I could think of any 2 features that I would really want on the next version of the IT, neither of those would have been anywhere near the cut. Probably not even in my top 10.

I can't say how I know, but soon enough others will find a way to confirm what I have said.

I don't know the situation with the slideout keyboard or the price increase.

See, it's amazing how different everyone's wants/needs are. GPS + nav app would be in my top 3.

In no particular order, my top 12:

1) native calendar (can be a cheapie one, people can get Dates or GPE Calendar for the full fledged one), including todo list

2) sync data in some fashion that is compatible with Thunderbird, Sunbird, and Firefox (bookmarks, contacts, events, todo's)

3) 2nd USB port, in host mode, charge via USB on the client port

4) universal "spell check as you type" from a built-in dictionary, that works across all of the native apps (notes, mail, browser, google-talk), and is available as in the API for add-on apps.

5) video out (pref. mini-DVI with DVI-A support so you can use mini-DVI to VGA and mini-DVI to video cables.)

6) support for bluetooth stereo headsets (with and without microphones)

7) slide'n'tilt keyboard (that can be tilted without sliding, to replace the built in stand), retaining the current buttons on the face of the device (not moving them to the keyboard)

8) Bluetooth PAN, and whatever support is necessary to transparently support storage from bluetooth hard drives (Seagate DAVE, Agere BlueOnyx).

9) software and bluetooth support necessary to use the N810 as a handset/controller for a cell phone (display caller-id, dial calls directly, dial calls from contacts list, answer calls, route voice in/out through N810).

10) finish the mini-mozilla browser, and add a mini-mozilla mail client (or, at least an IMAP client that delivers the functionality I keep talking about).

11) transparent to the app (except that it gets told to re-size itself to fit the new aspect ratio) ability to use both portrait and landscape. Doesn't have to detect via accelerometer, it can be button induced.

12) small user facing camera for chatting, camera/video (cell phone quality) camera facing away from the user (on the back of the device).

Since everyone's making their predictions, here's my equally pointless analysis:

First off, lessons learned:
Tapping into the Open Source ecosystem is one of the major goals for the NIT project. So killing support on the n770 so early was something of a misstep, since the time frame was insufficient for all those projects to emerge. Also, part of sound marketing on a new product release is reassuring those who own the old product of the quality of their purchase. That's basic. So I don't see them killing support for the n800 right away.
That said, something needs to be done about the n800's video speed. Screen tearing on scrolling is a serious problem.
Likewise, if they're going to have a camera on a device, they better support it.

Now, as for what _did_ get developed for the NITs:
so far, the #1 breakout application is MaemoMapper. The really cool part isn't the Navigation feature (that's done better elsewhere), but the ability (as an OSS project) to gain access to extremely high quality maps and photographs (and in the future, internet POI databases), and present them in a device you can have in your pocket. Any number of navigation devices can tell you how to get from point A to B, but how many can deliver high-resolution satellite imagery of your location?

So it's no surprise that Nokia would be developing a tablet with built-in GPS. That's one of the most active areas for open-source development, and provides one of the most unique features.

the other big usage coming up is mobile VoIP. Skype-enabled WiFi phones are a surprisingly growing sector of the market, and a NIT's cost is close enough for people to consider them. Adding slide-out thumbboard? I dunno, if it works, okay.

The final element they should be looking to add is excitement. OSS projects aren't terribly good at this, but even so, many of the most involved OS projects are to make the media players look and work in a useful manner.

As for MOSH, heh. Everyone's looking for the next ITMS, where a hardware company used their market dominance to build an internet service. Good luck on that guys.