osbock sent me a compiled hex for arduino ethernet (CS is digital 4) but it didn't work for me, here is the HEX so you give it a try as well.
http://dl.dropbox.com/u/18771283/ArduinoEthernetSD.hex
Don't forget that the sketch to be uploaded has to be in .BIN not .HEX and that this is a 4K bootloader so you need to change fuses accordingly.