I tried a standard transcend 1GB microSD card in my N800 and it works. It is very difficult to take it out from the external slot, however, as you have nowhere to grab the adapter. I posted a message about that a few days ago.

MicroSD of 6GB are available from Sandisk and 8GB should be available in a matter of days. At present, 6GB microSD is the biggest storage you can buy for the N810 as no miniSD bigger than 4GB appears to exist.