Ask HN: Freelance Matchmaker

I am curious about whether or not there is a freelancer-to-freelance matchmaking service, sort of like odesk or elance, but instead matches freelancers looking for subcontractors/help on projects with other freelancers.

If you're a freelancer, under what circumstances -- if any -- would you be interested in taking on a subcontractor/developer under the auspices of an experience or skill exchange? (i.e. You have skills with technology "x", and you'd take on a developer who was seeking to develop a skill set with "x") Sort of like a remote mentorship. Would there ever be a case where you'd do this with a stranger?