I think I read somewhere that we all mirror unconsciously to some extent (ever speak with someone with an accent and, to your horror, discover that you're starting to imitate or mimic it?) This tidbit makes me feel better about my T's mirroring when I notice it (the idea of them doing it on purpose strikes me as somehow manipulative).

I've noticed my T's mirroring most obviously when I'm in a good mood. If I'm feeling gregarious and funny, he becomes funny--he jokes, laughs, smiles, etc. Again, no idea if this is a consciously-done therapeutic technique, a totally unconscious way of relating (moods can be kind of infectious that way), or something in-between.