Good find Daniel2!
The important link is: http://www.roland-riegel.de/sd-reader/doc/index.html
This thread is titled "Compiling an SD/MMC and fat16 library" which I did not know about. I'm still looking for "SD card read/write with Arduino" which seems to be lost on the Google or Yahoo cache. The link above has files for a sd/mmc/fat16 library on a custom board with an atmega168. Its not designed around Arduino.... but at least its something to start with!
I think the "SD card read/write with Arduino" thread actually had code for Arduino but it has been awhile so I can't completely remember the details.