I don't know if this might have been mentioned here already, but last night I was revisiting the DVD, and noticed something (well, a couple or even three things really) for the first time. First of all, in the scene right before Jack's harmonica-playing moment in front of the tent, we see the boys in long shot, with Jack carrying a large piece of wood (driftwood?) on his back. I can't believe it didn't register till this late, but I noticed for the first time that Ennis goes up and pats Jack on the shoulder here. This further emphasizes Ennis's growing comfort and affection for his mountain companion.
Second thing: This news is currently making the rounds, and I just confirmed it for myself last night. The new tidbit of information is that Jack, in the first tent scene, can be heard to mutter "Fuck me" two or so times right after Ennis enters him. The words are spoken so softly as to barely register, but once heard, they are definitely present. The camera even rests on Jack's (actor Jake's) face as he utters these lines. When I first heard this news, the words somehow didn't seem right for the character in this moment, but they felt completely true and appropriate when I actually heard them. This further suggests Jack's worldliness (relative to Ennis), experience, and lust.
Third item: A friend of mine asked me to watch for a detail in the scene in which Jack returns to Signal in 1964. When we see Jack driving his truck down the road, on the sidewalk one can discern two old men (one handling a cane) walking together, going in the direction opposite from the one in which Jack is heading. My friend thought that this might illustrate the life that Ennis and Jack could have had in different circumstances, of growing old together, as lifelong companions. As I don't think any element in the film is throwaway, I certainly think my friend might be onto something here.