Hi!
In Dired, this should not happen any more (on OS X, using the "ns" interface). Emacs is now using a file-name coding system (utf-8-hfs) that converts the file names to non-decomposed names, before presenting them to the user.
However, in other contexts where decomposed characters are used, the problem is still present. The problem seems to be font-related. I tested all my installed fonts using `ns-popup-font-panel', and only a handful of them worked, mainly those that were provided by the OS: Menlo, Monaco, Courier, Courier New. Included in the fonts that didn't work were Vera and Hack.
-- Anders Lindgren