most likely on case-insensitive file systems, like the default HFS+ on the Mac and NTFS on Windows.

EDIT: to expand, reference lookups are filysystem lookups:

    01:42:24 (gh-pages) [bpowers@fina myproject]$ strace git show HEAD 2>&1 | grep HEAD
    execve("/usr/bin/git", ["git", "show", "HEAD"], [/* 79 vars */]) = 0
    lstat(".git/HEAD", {st_mode=S_IFREG|0664, st_size=25, ...}) = 0

EDIT2: formatting