My results from testing are that HFS+, the native Mac format, allows you to use different cases in file names, and will remember and display them that way. However, it does not differentiate between upper case and lower case when dealing with the resulting file name.
To demonstrate this, try creating a folder on your desktop called "Test Folder". Now try to create another one called "test folder". The case is different between them, but the letters are the same. At least on Tiger, it will not allow you to create "test folder", telling you that this name is already in use.