N800Canada, I am having the exact same problem you are having. I got my N800 yesterday from dell. After I charged the battery and made sure it was full, I installed the nokia update for OS2008. I did not put the sd card in before I updated the software. The computer I used to install the update is running windows vista. I noticed yesterday, after I put the SD card in (the nokia supplied card and adapter) that the N800 would not come back on. I took the battery out, put it back in, and then it powered up normally. As I used different applications, the N800 would lock up randomly. I then took the SD card out, everything worked fine; no lockups, N800 turns on normally.
To get to the point, my N800 is completely fine as long as there is no SD card in either slot. Once the Nokia SD card is in either slot, the N800 running OS2008 becomes unstable, hangs often, and will not power back on after shutdown until the battery is removed. I have other mini SD cards and tried them all with the same result, so I don't think it's the cards. Anybody have a cure? I've been searching forums all day...
Thanks!