Bahala na is described in "Basic Tagalog" as a Filipino philosophy, translated approximately as Happen what may! (Whatever will be, will be?)
Paraluman S. Aspillera, the author, says that "Bahala could have come from Bathala, the Lord Almighty to the Tagalogs.". Interesting that she said "could have come"; when I read that line, I thought she was saying that bahala wasn't a real word, so it may be a corruption of this other word. But that phrase, and the rest of her explanation, make it sound as if she is unsure of the relationship. I think she got carried away by her religion, turning a secular philosophy into an expression of faith, yet she still realized that the only reason for this theory was the similarity between the two words.
My Tagalog-English dictionary defines Bathala as God, but also defines bahala itself as trust or custody (it is a real word!), and na as an adverb of time, equivalent to the English 'already'. After her "could have come" she then translates Bathala na! as God already! or God will take care already!. I'd translate her version as God will provide!.
Though it's a possibility, she gives no justification for insertion of that t that turns trust/custody into God.
Anyway, she closes by saying...
You who have six or eight children and a stagnant income of P400 a month, what else is there for you to say these days but... "Bahala na!".