Ashley's SIL Michelle made this cake, which I think is super cute!  And she used a marshmallow fondant frosting recipe that you melt and pour over, instead of normal hard fondant:

Her post and recipe are found here: http://mmchandler.blogspot.com/2010/02/latest-installment.html

I also found this diaper cake on a website, though I can't remember where:

 I've made a few diaper cakes before, at various times, (for Rachel Callan, Tessa Egan, Suzannah Beesley, and Anne Waldrop):

But mine obviously weren't as big or extravagant as that one!  Crazy.