Some people have been working on a modification to allow them to use the official arduino ethernet shield with the provided yet unsupported SD card reader (there seems to be major updates to the shield: as I write these lines the NYC maker faire is on and the arduino team is announcing some major changes to the arduino family including, I hear, a new ethernet shield...).
I will modify my ethernet shield to allow me to use it stacked with the VS1053 chip mp3 decoder shield and not have them both fight for SPI communications (this would happen because the Wiznet chip that comes on the ethernet shield, does not implement a proper version of SPI and it is recommended to disable SPI on the chip altogether when not in use - basically it disabling the SS line doesn't actually stop the chip from sending information to the SPI bus, confusing all the other SPI devices that happen to be connected to the bus).
So the steps I will follow are:
1. add a wire between the SEN pin of the wiznet chip and one of the ATmega ports on the arduino (I choose D8).
2. modify the ethernet library to change the state of the extra pin every time we finish a SPI read/ write cycle.
I will keep posting as I go...