Are you doing this on the MyDocs partition? That is a Windows FAT32-formatted partition (so that it can be exported to any computer you connect the device to) and does not support case-sensitivity.
/media/mmc1 $ touch fOo /media/mmc1 $ ls f* fOo /media/mmc1 $ mv fOo foobar /media/mmc1 $ ls f* foobar /media/mmc1 $ mv foobar foo /media/mmc1 $ ls f* fOo /media/mmc1 $