Wonder no more. It's going to move more of your weight towards the back wheel, although the effects in terms of overall weight distribution are probably negligible. Here's a fun game for a rainy day: take a bathroom scale (or two), sit on your bike and weigh the front wheel and the rear wheel. Switch from a straight seatpost to one with setback, repeat the process and see how it affects things. I doubt much will happen.

FWIW, "ideal" front:rear weight distribution is usually around 45:55 or so.