I would really send it back and get a replacement. The internal sd slot is quite important (swap file etc).

Ironically, a week ago when I got my N800, I did the same thing. I put in my 1Gb card in the external slot and it was quite dirty and the internal slot stopped working. Cleaned the sd card with spit (not the most hygenic of solutions but I'm 15 :P), wiped it down, put it back in and it still wasn't working. Wasted an hour feeling pissed until I read the getting started manual and it said you needed the cover on .