On Vicarious Therapy the author wrote an to my opion already classic post on "blog therapy".
She explains her ideas about using a blog during psychodynamic psychotherapy. She has given the subject a lot of thought and wrote the pros and cons on different approaches for using a blog with therapy. Be sure to check the comments.
I believe a blog is an incredible way for the patient to clearly layout their defense mechanisms and for the Psychiatrist to have a much more in depth and detailed view of his/her patient's transference. If the blog is two way; one written by both the patient and the pdoc, I could see tremendous value for the therapist to also recognize their countertransference more clearly.
And this is just the beginning of her post.