I don't have a suggestion for the diaper cake idea, but I do have a suggestion for a diaper wreath. My girlfriend gave me one when my last baby was born and I loved it so much I made one for my sister-in-law who is expecting her first little girl (after 4 boys) any day now.
You will need a wire wreath form. Open the diapers slightly and sandwich it around the wire wreath form. Use a tiny hair rubber band to scrunch the diaper up a little and help it stay on the wreath. Go all the way around the whole wreath. Then use curling ribbon to cover the rubber band and also add additional support to keep the diaper on the wreath. Then use some curling ribbon to attach "baby items" around the wreath. I used diaper rash ointment, pacifiers, a bottle, a bottle of baby oil and baby lotion, a brush/comb set, etc.
The fun part is to find diapers that might match the color scheme. For instance, I found that the Walmart brand White Cloud has purple diapers. These are what I used to make the wreath for my sister-in-law. But Pampers Swaddlers are yellow and Pampers Baby Dry are green, Huggies have blueish colored diapers. Anyways, you can use a different brand to make a more "girly" or "boyish" wreath or one that is more unisex.
(sent in by email)
I have been looking also. I have found two types. Try craftbits.com or wrappersandmore.com. Both sites have good instructions. Good Luck.
Editor's note: Here's one on ThriftyFun: http://www.thriftyfun.com/tf35522077.tip.html
Here is another great site, for diaper cakes or wreaths:
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