This was originally written by Bianca when she owned the fanlisting, before the release of At World's End; Amber kept it up because it "is actually incredibly accurate regarding the third movie. It has some good insight into Tia's character and the theories that were spewing everywhere after Disney released Pirates of the Caribbean: Dead Man's Chest to the masses."
I am keeping it up for the same reason.

And so...
Ever since it was revealed Tia was going to play a major part in third movie fans have speculated the likely and the plain absurd about her character. The "history" between Jack and Lord Beckett has been released as well, which only fueled the theories about Tia. Jack worked under the employment of Beckett, but when he ordered Jack to deliver a group of slaves to the Caribbean, Jack refused to go through with the heinous task and set them free. Jack's act resulted in being branded a pirate by Beckett, and also the destruction of his merchant vessel, The Wicked Wench. Determined to reclaim his ship, Jack swam to the bottom of the ocean, foolishly thinking he could drag it out piece by piece. Nearly drowning, Davy Jones rescued him and after making their 13-year deal, raised the ship from the ocean. Jack apparently titled it The Black Pearl at that point.

It is very possible that Jack set the slaves free near Tia's home, meaning the men and women holding candles and mourning the loss of him were in fact that group of slaves. This would explain a couple of things, including the reason why Lord Beckett wants Jack's compass, since Tia gave it to him (possibly around the time he released the slaves).

Another highly and most likely true theory, is that Tia Dalma was the oh-so certain lover of Davy Jones. Since they own the matching lockets and Tia spoke very knowingly about the woman "as wild and untamed as the ocean," there's little doubt on this one.

There is of course the matter of why Barbossa was hanging around her shack. In the scene where Jack gives her the monkey we see the animal run towards the back of the hut and onto a pair of boots, and it's quite obvious Barbossa is comfortably resting as if he's made himself at home. It is possible Tia foresaw the fact Jack's crew would need Barbossa to guide them through the oceans, thus brought him back to life. It's also possible the two of them had a former connection, and resurrecting him was personal.