(Incidentally, PG designed the /jobs posts to be the one privilege that YC startups get on HN. Other than that, the site policy is to treat them the same way as everybody else.)
It's not uncommon for posts to be in a grey area between job ad and story. A programming test for an interview process doesn't seem all that grey, though, so I've been killing these. Sorry to be a hard-ass about this, but I don't want to be accused of enforcing the policy erratically.
Edit: I'll unkill the post (but otherwise leave it demoted) so the discussion can remain open.
Edit 2: The argument has been put to me that this is newsworthy because A16Z are doing it. Since that's corroborated by external sources (e.g. Twitter), and the post has gotten a lot of upvotes even while it was buried, that's reasonable—so we'll restore it as news rather than as an ad. Besides which, HN is one of the few places where a programming test counts as interesting.