The Mad Priest and our mutual friend Goran have a scoop over at "Of Course I Could be Wrong." They point out that:
1. The Church of Sweden's leaders have said that they are willing to allow gay people to marry in church on the same basis as heterosexual couples, although bishops are unsure whether to call the unions marriage. The decision will make the church the first major denomination in the world to allow full gay church marriage in practice.
2. Under the Porvoo Agreement, The Church Of England is in full communion with the Church of Sweden and allow each other's clergy to work in each other's parishes in either country.
Now the Church of Sweden is not a member of the Anglican Communion, so it's relationship to Canterbury is not precisely analogous to ours.
[Here comes the "good question":]
Still, it seems to me that Rowan Williams needs to explain why the Episcopal Church's decision to allow the blessing of same-sex relationships is, in his eyes, a Communion-breaking offense, while the Church of Sweden's decision to allow same-sex marriage is not.