> But most applications (and my brain) don't really want the rules to
> suddenly change on them in the middle of the file system...

Think of an NTFS, HFS or FAT32 disk that you share with another OS in 
which you dual boot your machine from time to time.

Those filesystems have to provide this semantic somehow, or you will get
inconsistencies on disk. It's not only a case-insensitive view of a basically
case-sensitive filesystem. Some names become invalid on creat() if you
are looking up without case, that are otherwise valid.

If the implementation as a mount flag is the proper way I don't know.

Samba just was an example of something outside of that limited scope that
could benefit from it. There would be other, far more interesting ways
to speed up Samba (if that would be the primary goal, I'll start with putting
parts of it in the kernel, like the nfsio threads).

Martin