That bit where Ianto pulls away? It got me thinking more and more that it's not the question of when they started shagging that interests me, it's when Ianto let himself be present. You could posit that they started something any time from this moment on and make it believable, but emotional involvement on Ianto's part is an entirely different matter.

This is just absolutely spot on. I would assume it was somewhere between TKKS (where he was playing a role, I think) but before we got the Brokeback Coat scene, although possibly he never revealed himself to Jack. Hell, it's possible it was TTLM.