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.