Thanks everyone for the interesting input, I knew I'd get some great info! The reason the OP on IMDB had a compelling argument was that he noted that specific dates were mentioned, either in PA annoucements or painted signs, that basically stamped the dates on the scenes, and therefore made it biologically impossible for Jack to be Bobby's dad. I thought I noticed the same things on prior viewings. So, that posters argument was simple: either this was intentional, and Jack was very subtlely implied to not be the sperm-donor, or else, it had to be a mistake.
So let's debate that last question: first of all, was it a mistake? I guess we all would hate to think there are mistakes in an otherwise perfect movie, but we all are well aware of disappearing logs on stumps and the magic levitating peanut jars, so, yeah, it happens. But, why make a point of making sure these dates are announced in the first place, if it was all a mistake. It just seems so un-Ang-like for Ang, who we all know is meticulous, to take the trouble of making sure the dates are included in these noted scenes, and have them be wrong chronologically. Was he secretly putting in a little puzzler, to be discovered by only the true Brokie? These dates, as I understand, were not a conflict in the short story, so it was purely an issue of the movie.
And, let's just say, for argument, that Jack was NOT the father of Bobby. Some of you wonder why in the world this would be added to the plot line at all, when it was not developed in any way, and it could have made a bigger impact on the whole theme of the movie? Well, again, the points are subtle: as noted by me already, Jack seems more disconnected to his son than Ennis to his daughters. A poster above made some excellent points about why that might be, but Jack seemed just so much more available and willing to leave his family, namely his son, for Ennis, and I'd like to think that he would be just as loyal to his son as he was to Ennis.
Another small but little-discussed detail: did it occur to you that Jack's parents had NEVER met their only grandson? Jack always visited Lightning Flat alone, and even in the book, Lureen specifically says she never met her in-laws. I can imagine that the distance and their limited means might make travel to Texas impossible, but not a mention by them, not even a picture of their grandson in their house (although, no picture of Jack either).
So, mistake, or very sly insertion by our famed Director, to add this other element of Jack's miserable life, that he was stuck in a marriage of convenience and appearances only, and his son was not even his son? Just makes it all the more sad, that Jack then died so bitterly, sadly ...... alone. He lost Ennis, his parents were distant, his wife didn't give a damn, his in-laws hated him, and he had no kids, no legacy to continue. Makes it all the more profound of what Ennis meant to him.