Probably easiest not to change it then? Ok. 
[EDIT]
Am I correct in thinking that the 3.3v rail is from the USB port?
That will mean that Arduino needs to be USB powered to read from the MMC card since it doesnt like 5v.
I also forgot to buy a multimeter which means I cant test it. Damn.
[/EDIT]
[EDIT]
Will I need pull up/down resistors?
This schema for a PIC shows resistors on the data input pins.
http://www.captain.at/electronics/pic-mmc/mmc.png
[/EDIT]