First, I ask that you read Vatican II's (short) decree on social communications, Inter mirifica. Then realize how woefully out of date it is. Time for some aggiornamento ("bringing up to date"), no?
Second, read Fr. Z's post on an article in The Catholic Herald about Pope Benedict, priests, and blogging. (See also here.)
Third, read Pope Benedict's recent message for World Communication Day.
I'm not saying the Magisterium needs to expend time and resources and energy on some official document about clerical blogging — I think the Pope's recent comments suffice, really — but I am curious how many Catholics recognize this as an actual fruit of the Council, as a positive meeting point between the Church and the modern world.