Specifically, :OS-PROVIDES-POLL isn't on *FEATURES* there because :OS-PROVIDES-POLL really means "OS provides poll() good enough for SBCL's internal needs", which includes the ability to poll on things like stdin and /dev/null -- which Darwin's poll() doesn't do.
I'm also a bit dismayed by the amount of SB-IMPL::FOO going on in DEFUN POLL. That stuff is going to break, probably sooner than later.
So SWANK could probably use Darwin's poll() just fine -- except some of the SBCL internals it tries to use aren't there.
Not sure what's the best solution -- and what's the most expedient one.
Cheers,
-- Nikodemus