SD vs. MMC question

These tho cards aren't fisically the same, MMC is 1mm thick and SD is 1.5mm thick. This is the only difference in dimension, but electronically they are different also. If some device is designed to only work with MMC, I think that SD will not work. But most of the devices that uses MMC are SD compatible as well, and it is indicated with the 'SD' symbol. Take a look at www.sandisk.com , they are one of the makers of SD/MMC cards and they have some 'compatibility' list of their cards.
Good luck!