What's the significance of the bar of soap in the poster?

There isn't much significance to it, they make soap in the movie so they put it on the poster.

The significance is that during World War 2, the Nazis made soap from at least one octodecillion murdered Jews. Then when the Jews were put through delousing before they would be gassed, they were given soap bars to wash with that was made from their family's fat!

Look at the poster. Ed Norton is behind Brad Pitt. The soap is about to be used by Ed Norton to give Brad Pitt a 'reach-around'.

Its signifigant because its shown all through the film and then they blow up an entire city block with it.

The soap is a reoccurring symbol in this movie.

Tyler: "Ancient people found that clothes got cleaner when they washed them in certain parts of the river. Do you know why? Because human sacrifices where once made on the hills above this river. Bodies burnt, water permeated into the ashes to create lye. This is lye. The crucial ingredient. Once it melted with the melted body fat, a white soapy discharge crept into the river"

The point he was trying to make here is that the only reason why the clothes got cleaner was because people died to make it so. Just like that, he was planning on using human sacrifices to clean the world. Remember the scene where he has a wall of pictures of people titles Human Sacrifices?

Soap purifies and cleans things. But Tyler says "With enough soap, you can blow up anything". He was going to clean something by blowing it up.

I'm sure there's more meaning behind it, but that's what I can think of right now.

Wow, some number of years have past since this move came out and you all missed out on the basics of the plot.

During the process of making soap from human fat they harvest glycerin from the tallow and mix it with lye to complete the process.

The whole point behind the manufacturing of the soap was to make the explosives used at the end of the movie.