Correct use of the comma.

The first form is accepted by the Associated Press, so it's used in print journalism and most informal or semi-formal writing.

The second is the one I was taught in elementary school, and I see it more often in formal or older writing.

I use the second form more often, but I think that's just because I'm in love with the comma. Also, like butterfli's situation with the will, it can sometimes be confusing if you don't separate the last two.

Side note: I hate when the last two things are really a pair, so I say "John, and Bob and Margaret", and my teachers mark off because "it's one series". It's not, you dolt. T_T