I used to have a super thin (officially 7mm) and cheap phone, I almost completely removed/dismantled the cell phone further and stuck it at the back of the n800 (with its battery cover removed also) and had a n800 white silicone case over both devices (I could easily read who was calling). The net bulge was about 2 mm. Almost made me think I had one device :P

This is amazing to watch, just wish the specs of the n810 were comparable to the n900.