Heh, I don't think there's that much retcon involved. I watched Season 1 from the beginning with the assumption that Jack and Ianto were shagging, simply because I knew a lot of people were shipping them (but aside from his name, I knew nothing else about Ianto). I did doubt my initial opinion after I saw Cyberwoman though-- it wasn't until I saw Fragments that I again began to believe that they were shagging throughout the entire S1-- but it didn't seem like ret-con to me, it seemed like everything fit into place better.