The music player can access it too. I took the 16GB card out of my old phone and plugged it into the Atrix. It immediately recognized the 7GB's of music I already had stored on it.
However, I did find that if I connect to my computer with the USB cable in Windows Media Sync, Windows Media player on my computer only saw the internal memory. I switched to USB Mass Storage and it saw both the internal memory and the sd card and my music sync settings for the card were still set up.