"Jack I swear" is the last line of dialogue in the novella too. It allows the reader or viewer to fill in the blanks. I like to think that Ennis is saying "Jack I swear:"
1) I will always love you
2) If I had known what I know now, life would have been different
3) You will be the only true love of my life
The "Jack I swear" line and the cherishing of the shirts demonstrate Ennis' emotional development towards self-acceptance.
On another message board, someone posted a brilliant analysis of the last twenty minutes of the film, comparing it to Virgil's The Aeneid. I have not read it, but apparently at the end of the book Aeneas (Ennis?) sees the ghost of his late lover Dido and exclaims, "I swear by every oath that Hell can muster, I swear I left you against my will. The law of God--the law that sends me now through darkness, bramble, rot, and profound night--unylielding drove me; nor could I have dreamed that in my leaving I would hurt you so."
By the way, the National Gay Lesbian Task Force is recognizing Annie Proulx, author of Brokeback Mountain, at their awards dinner next week. I will be there and I am looking forward to seeing her.