Freetonics says about the official ethernet shield: “The W5100 has a design flaw that causes it to maintain control of the SPI bus even when another device has been selected”
Someboday knows some hack to solve this?
Edit 1
I think I figured out the problem and it is that The standard Arduino shield uses the Wiznet5100 chipand this chip has a flaw in that the CS signal does not tri-state the MISO line. Has a look here: john.crouchley.com/blog/archives/662
I hope we can fix it so we may help others to do the same.