I just now tried it by doing a live boot with Ubuntu (9.10 if it makes a difference). I made a new file on a FAT32 formatted USB drive, "Readme.txt". Then I made another new file and failingly tried to rename it to "readme.txt" but got the error
[B]The item could not be renamed.[/B]
The name "readme.txt" is already used in this folder. Please use a different name.
It seems that the reason for this is because the file system itself is one that is native to Windows (like the ones you quoted from Wikipedia: VFAT, FAT32, NTFS). I say this because I then tried the exact above procedure on the local Linux file system, and I was able to have both files, readme.txt and Readme.txt, in the same directory just fine!