Interfacing SD/MMC cards to arduino is a must, if you didn't need it soon, you will do later. Myself being new in arduino's world i had to google for an implementation. i came across this post where i got a simple schematic to interface the SD/MMC cards and few codes reading 12 pages of furstrated people who got no luck , i used "sd_raw", ufat and another one more code i found in arduino playground and none of them worked for me, I used the schematic on arduino forum with same resistors ratios but not the values, the schematic is composed voltage deviders to supply the SD card with no more than 3.3v out of the 5 volts the arduino is using.
For few days i didn't got further away from the other flocks (in the forum), till i came across SDfatlib from which worked just fine from the first trial on my curcuit.
The great thing about it is that it supports FAT16 & FATE32 and supports read/write in raw format. the liberary is also stocked with a large number of sketches which contains pretty much everything you want to do with SD card.
Schematic was taken from here.