some people have sent pics through email and had it work apparently. there was a thread about it here ages ago.
It never worked for me.
SD cards are dirt cheap, about $30 for 2GB, and you can get an adapter for your computer for about $9.
But besides pictures and backing up VC games there's not much you can do with them on wii
(oh i forgot excite truck uses MP3 files on SD card)