Docedison:
Bypass capacitors, sounds like the Arduino finds the card... Perhaps reading anything else is problematical due to lack of bypass capacitors... .1 and perhaps a local 4R7 on the 3V3 side?
Are those needed though? Looking at the Ethernet shield schematic, there isn't a cap on the 3V3 line anywhere near the microSD, nor is there a resistor on it.