Sorry for coming late here, but a few comments:

- the N8x0 (like any modern gadgets) have their own built-in regulator, so it would be ideal to avoid needing a second external regulator.

- I read the Nokia 2mm spec, as linked above by Fontus, allows up to 9.3V, so a 2-cell Li-Ion battery should work fine. However it appears that Nokia do not follow their own standards. Connecting one (7-8V) results in a "not charging" message on my N800 and on my Nokia phone
Note that this does not damage the tablet - it simply refuses to charge.

- But 5-6V in does work on my N800 and phone. A 4xAA NiMH pack can be directly connected to the 2mm Nokia plug.

- to enable hot-swapping internal batteries, you could add a capacitor inside the NIT across the battery terminals. e.g. 0.1F 5V. A software hack to make the screen stay off when battery cover is open would help.
Might need a small resistance in series (if lacking Ri) to limit current when the new battery is inserted.

- if you do want a cheap, light 5V regulator, see http://www.dealextreme.com/details.dx/sku.29208
(has micro-USB plug, so needs cut/solder the cable)

- you could add one or two 18650 cells in parallel with the cell of the standard battery, and attached to the back cover. I don't know how well the battery electronics would cope with re-charging, but it would be slow at best. You might need an external charger.

riderman, where do you get the <50% efficiency from?
I'd have thought 4xNiMH, or 2xLiIon+switchmode reg would do much better. Are you assuming the N8x0 uses a linear regulator?

That guy using a 7805 to convert 12V->5V made me shudder.

> Does someone know, at which voltage of the internal battery the N810 shuts down?

The battery has a safety cutoff (~3V?) but the tablet will shut down before that happens. You could just run the device flat, then measure the voltage. Why do you ask?