Okay, I'm new around here..but I'll just jump into the conversation.
I've always seen Similitude as the key TnT episode. I think there is very much attraction and deep feelings there, but T'Pol certainly doesn't realize she's in love until the events of Similitude. And its not just from the conversations she has with Sim, but the fact that she loses him - and he is essentially an analog to Trip. That had to have evoked pesky feelings of love in her.
Harbinger is not my favorite TnT episode, but it does make sense to me. She's behaving immaturely, because she has no idea how to react to her feelings - the feelings she's been aware of since Similitude. She's ashamed of them, and Trip can't possibly understand it because he wasn't around during Similitude, isn't a Vulcan and has no idea she's on the pipe. Now, the romantic in me, I wish it had been written differently, but there you go. They are two people, from two different cultures in a time of war - they both aren't seeing things clearly. She might be cold at the end of Harbinger, but he's awfully clueless for not recognizing how unusual her behavior has been.
Thankfully, I believe that starting in Forgotten, things are going on behind the scenes that are much better written and have the two behaving like grown ups. At least in my own head. That's my story and I'm sticking to it.
As for the bond, I tend to think it forms gradually, over the course of the relationship in the third season - not just during Harbinger, since the conversations they later have are probably just as intimate as what went on during her "exploration". Which I also imagine was far more intimate than either of them admit at the end of that episode.
edited to add: I don't know when Trip falls in love with T'Pol, but I think it is sometime between The Xindi and Similitude, probably gradually. They have several flirtation scenes, so its hard to pinpoint. But by the neuropressure scene in Similitude, he's treating her like his girlfriend, whether he admits it or not.
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