The list has however oozed out, and it is (drumroll please) Canada, Australia, New Zealand and Japan. The US was invited to join but refused to give reciprocal arrangements. The same is possibly true of South Africa. Hmm. What do that list of countries have in common? Tricky one.
Beyond that, observe that the scheme is talked about as being 'points based'. Now look at the points system
You will observe that you need 50 points, but you get 50 points as follows:
30 - Having a passport from an eligible country
10 - Being between 18 and 30
10 - Having 1600 quid
This is, of course, not in any meaningful sense a points based system, since you need 50 points and can only get 50 points by meeting 3 criteria. However, 'points based system' has a nice ring to it as in the world and its mildly xenophobic great-aunt saying 'we should have a points based system like Australia'.
Edit: it is clear that the list of countries has been decided but the news story is embargoed until Tuesday (hmm, good day to bury bad news?), but it has leaked out, rumour has it via the Foreign Office. Here is a sample story http://www.globalvisas.com/news/uk_working_holiday_visa_rules_reformed549.html