My earlier blog post was about the role that faith plays (or doesn't play) in my own politics. I didn't have a full answer on the specifics of Archbishop Rowan Williams comments, apart from tentatively suggesting that it's OK for bishops to make political points.
However this blog post is a very good summary of the case for the Bishop getting involved and in passing points out the contradiction at the heard of Cameron's Big Society: those who he praises as the heart of the Big Society are in many cases extremely unhappy with what he's doing:
"If Christian groups are to be expected to help fill the gap left by state withdrawal, then Archbishop Rowan clearly has a right and a responsibility to question Government initiatives...
...the Prime Minister and his party cannot promote the idea of a cohesive, communal ‘Big Society’, and simultaneously dismiss the views of the leader of a group which helps maintain that society as ‘plain wrong’, ‘unacceptable’, and even according to Roger Gale, ‘offensive’. The Prime Minister cannot have his cake and eat it too."
It's well worth reading in full.
Note - I don't think it's a good idea for faith groups to be replacing state services for a wide range of reasons.