Conjecture for discussion: This is true of almost all aspects of running a successful software development group: Hiring, Project management, Architecture, UI Design...
Nobody seems to have "figured it out" to the point where you can write out an objective, repeatable process. It almost always comes down to having people with good "instincts" (i.e. They make good decisions) in positions where (a) they can make decisions, and (b) the team acts on those decisions without obstructing them or screwing them up.
I would argue that that is a sign that it's been figured out. Putting candidates before interviewers with good people skills is an excellent filter. Interviewer training definitely helps (at least it helped me get better at conducting interviews and seeing past the happy face candidates put on), but good instincts are a powerful tool.
(Anyway, can anyone point to a company that truly figured out hiring? From what I've seen, it's mostly down to decision makers having good people instincts. Resumes, references, cover letters etc only get you so far. They aren't even that useful as filters, since they often leave out good candidates as well as poor ones.)