Sorry to reply to my own post.
This is an OpenJDK-related issue, I think; when I use Apple's java, I get :UNIX and :DARWIN in *FEATURES*. I'll try and investigate this further.
For OpenJDK 6 on OS X, System.getProperty("os.name") returns "Darwin". So isPlatformUnix in Utilities.java needs to be amended. I imagine this will fix the problem.
I'd be happy to file bug reports and add/amend documentation to the Trac system if someone would make me a common-lisp.net login (or does that come from elsewhere?). Or would people prefer patches? If so, what's the preferred format?
Cheers,
John :^P