Working with MMC/SD cards can be frustrating. I had a hard time making my first effort work..
Are you using an MMC or an SD card? Have you tried different cards? The schematic you're using is good, you can cross-reference your work with a different version which shows the wiring for an SD socket here http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1206874649/1#0. The main difference is that this version powers the card from an Arduino's 3v3 supply whereas the version you based your circuit on is using diodes to drop the 5v to around 3v3. Other than that both card types can be addressed in the same way.
I've come in late to this conversation - what do you want to achieve? What code are you using?
I'm already using the schematic without the two diodes, but I tried that one too.
I really don't know how to find the problem...if it was a SW I could use a debugger, but here?
Anyway...I'm using a Kingston 64mb SD. The card works fine in my PC even after all the soldering I've done on the card itself (I can not find a socket...) so the problem is not there.
I checked the value of the resistors and thery are all fine...
Connected VCC on SD to 3v3 on Arduino, GND on SD to GND beside 3v3,
pin 1 on SD to 10 on Arduino
pin 2 on SD to 11 on Arduino
pin 3 is GND
pin 4 is VCC
pin 5 on SD to 13 on Arduino
pin 6 is GND
pin 7 on SD to 12 on Arduino (without any resistor because it's from SD to Arduino so it's already 3.3v)
Am I forgetting something? 
I'm trying to build a simple 3-channel datalogger...the input part is easy but useless if I have no place to store the data I collect...