Update: The recent Mac OS X.4.9 update fixed me right up. Go figure.
I've been using the HumaneText OS X Service for years and have never had any trouble with it. However, earlier today I tried to use it to convert something from Markdown to XHTML and nothing happened. The menu is accessed (i.e., it flashes), but the text is not converted and the highlight is removed.
Any ideas? I've tried everything. All of my other services work without issue and any new services I install work just fine. I'm stumped.