One answer to this question mentions the UNIX 03 certification of OSX. Now AFAIK the standard file system of OSX is/was HFS, which "saves the case of a file that is created or renamed but is case-insensitive in operation" (i.e. it's case-preserving but case-insensitive).
Does the UNIX certification or POSIX require a case-sensitive file system?
Best Answer
According to the POSIX specification:
--snip--
--snip--
So it looks like case sensitivity is the norm, but it is possible to support a non-compliant (case-insensitive) file system and still call your product UNIX as long as it can also support case-sensitive file systems.
(edit)
Actually, see this part of the specification:
So it looks like is purposely left ambiguous - it is neither explicitly permitted nor forbidden.