I just walked out of the theater half an hour ago, and someone in that scene definitely says "I'm sorry." I listened very carefully, because this dialogue came up in another thread. I think it's Jack that says it, because I'm pretty sure Ennis doesn't say anything.
So, if it's Jack that says it, what's it mean? "I'm sorry, Ennis, because I know you don't want to be pulled into this, but i love you too much to just let you go." What immediately precedes this scene is a mystery (unless it's described in the story -- which I still can't get). When we arrive in the movie, Jack's already in the tent, moving around a lot -- I assume undressing. What's happened just before that? Has Jack said something like "I'm going into the tent -- you can come in if you want." It's clear that something has happened because Jack is in there waiting, and Ennis is outside struggling with the decision. Knowing what it takes to draw a conversation out of Ennis, I can see Jack saying he's sorry for drawing him back into the tent. Jack's the instigator here. He understands that they're embarking on a path that's going to be very difficult for Ennis.