I'll preface this by saying I'm far from an expert in the field, but as far as I can tell there should be no issue having SDXC cards recognised by the N800 assuming they're formatted to a compatible filesystem (and they can be formatted to FAT32 [if you use Windows like myself] with a bit of messing around).

I've almost convinced myself enough to buy an N800 and a 128GB card, because the utility would be beyond fantastic. If I do, I'll report back with confirmation.