I have exactly the same issue as promethh.

I have an N810, a 2GB MicroSDHC and just bought a 6GB Sandisk MicroSDHC. I used the same micro SD to mini SD adapter for both cards.

1) The 2GB card works fine with the N810 so there can't be an issue with the SD slot on the N810 or the micro to mini adapter.
2) The 6GB card works fine on XP, Vista and Linux; I transfered are read files from the card just fine.

When I insert the card in the N810 I get the "Memory card corrupted or unformated" error.

I tried formatting it through the N810, the formating box comes up but does nothing and quits after a couple of seconds.

I formated the card FAT32 through the various operating systems, nothing.

I have used the Panasonic SD Card Format utility to format the card but still nothing.

Any ideas?
I bought the card to take on a trip with me on Sunday but it seems I won't get to use it. Please help!