^Actually Forgiveness wasn't the first tale about the origin of the transporter. Marvel Comics did an issue on that subject in 1980 (a story which was actually a bit similar to ENT's "Daedalus," in that it involved "ghosts" that were incorporeal victims of early, failed transporter experiments). Also preceding Forgiveness by a couple of years was "Dead Man's Hand" in the Pocket anthology The Lives of Dax, which had Tobin Dax and Sarek's father Skon perfecting human transportation during the Romulan War -- which ENT has since contradicted, of course.
Ack, I shouldn't have said "first" origin tale, just "an origin tale", though I didn't know of the two others. I should have just assumed it.