Couldn't find any info, and it seems too resistant to use brute force... How is it done?

Open stand.
Look at the back side of the N800. There's a little nail-slot there.
Open. It opens from the 'inside', i.e. it's hinged at the corner side of the N800.
The card goes in with the label side down (but it goes in only one way anyway).
No brute force needed..

Yeah, I see the hinges... But then, which way to pull it? Do I pull it "away" from the tablet, or do I pull it downwards (imagine the tablet lying down).
It seems there is at least some need for force there...

Ok, so I got it open... It did take a fair bit of force, but it opened perfectly, and I think nothing broke... But still... it took me like half an hour of reverse engineering

It is a bit of a cheap flimsy lid but it works. It is quite different from the 770 one. While the 770 lid popped out and swung in a plane perpendicular to the screen, the N800 one just simply swings up in a plane parallel to the screen. It is the same motion you open the lid as you would do to pullout the card with your fingernail since there is no spring loaded release anymore..