I made a diaper cake for my sister's baby shower last month and it was a total hit. Here is the picture:
I've also made one for a baby boy:
I use huggies size 1 diapers and don't use glue or anything to to keep them together. I just sort of roll up and hold together as many as I can then tie a string around them to hold them together. I just keep adding the groups together and holding them together with more string. When it gets big enough, I use baking dishes for molds for each tier. I eventually cut out any extra string except for the very outside of the layer, and that is hidden beneath the ribbon. Then I just tuck in lots of goodies like Boudreaux's, J&J baby wash, some pacifiers, and I top is with a soft plush rattle.