I'm going out here on a limb, but I'm guessing that Jacob is Desmond's father, as well as the individual listed as "108 Wallace" in the Lighthouse.
In The Subsitute (6x04), we learned that the candidates are here to replace Jacob and that all of the ones written on the cave ceiling have the numbers (4, 8, 15, 16, 23, 42) in front of their name. We also know that the show has always made a big deal about the numbers adding up to 108. (The obvious example being that every 108 minutes the numbers must by typed in and the button pushed.)
We learned in the Lighthouse (6x05) that the name written next to #108 was someone named Wallace. Given the signficance of #108 and what little time there is to introduce a whole new character into the show for us to care about, odds are Mr. Wallace is someone we've already seen. Virtually every character in the story has a first and last name, however Jacob would certainly be a notable exception.
Speaking of Jacob, I wanted to touch on Desmond.
Before Desmond broke off his relationship with Ruth, we don't know anything about his past, beyond that his father "died" when Desmond was young and he spent his time taking care of his brothers rather than going to school. Given that the show has gone to great lengths to show how unique and special Desmond is, I think there is a link between Jacob Wallace and Desmond David Hume.
I believe that the reason Desmond is unique and miraculously special is explained by him being Jacob's son.
Mind you, this is a wild ass guess, baced on Wallace being a scottish name and the lack of info on Desmond's past, but it seems like a reasonable wild ass guess to me. ;)