I'm new to this too, but I think if you're have 3.1.3 then you can't jailbreak your iphone 3gs yet. And even if you could, if you shutdown your phone you'd have to redo the blackra1n thing again
I have 2 questions.
1) I'm thinking about buying an iphone 3gs, if I were to buy one would it be 3.1.2 or would it already be 3.1.3 right out of the box?
2) If I were to use blackra1n to jailbreak it, what are the chances of bricking it?