I have no idea what the technical definition of attachment is, but I've always felt an attachment for my T's. I guess for me that just means I trust them, I'm comfortable with them, etc. I've never had attachment problems so it has never really come up as an issue in my therapy.

I would think it is important to have a degree of trust and comfort with your T in order for the best therapy results, but I don't think making progress in therapy is totally dependent on a secure attachment (as evidenced by people here on PC who struggle with attachment issues of many varieties yet do make progress as they work toward better attachment.)