Sorry everyone if you thought this cake is edible. Its is not :( Don't be disappointed; after you learn how to make this diaper cake, you are (one day for sure) going to make some expectant-mom so happy that you would come back to thank me :) Sorry about the long post, for, I want to do complete justice to this beautiful experience of making this cake for an expectant mother.
Baby Shower, Seemantham, Godh Bharayi are a few names for the day we celebrate and bless the expectant mother and the fetus. As the  expectant mother eagerly awaits her babies arrival, friends and family throw her a party, perform pooja or other customary things according to ones culture in her 7th month of pregnancy. Gifts are given to the new mother which mostly comprise of things for the baby.
In our family, friends and relatives buy small gifts for the mother-to-be and arrange a small table filled with gifts. On this occasion, Diaper cake acts as a center piece. Which expectant mothers eyes does not mist after seeing this thoughtful gift? I made this when my sister in law was pregnant and she loved it! 
You guessed right; Diaper cake is made of Diapers. The below instructions are for making a 3 tire diaper cake.
Ingredients:
Infant Diapers: 80 (usually 1 pack comes with 100 diapers). 
Scissors
Elastic bands: 1 pack comprising of big and small bands (about 100)
1 inch width colorful lace: 3 different colors 2 meters each or same color 6 meters. 
1/4 inch lace: same as above
Teddy bear: small one or any other stuffed toy
Feeding bottle: 1 big and 1 small 
Lots of time (at least 5 hours): I split between 3 days but the effort is completely worth it.
Optional items:
You can also decorate the cake with bibs, booties, pacifiers, socks, mittens, comb and other baby items. I personally did not include these because this would mean I will have to unwrap the items from their cover; after unwrapping these its not hygienic to give the same thing to the baby. If you end up finding some good looking stuff for cheap which the mother will not use anyway, go for these.
Instructions:
1. Unpack the diapers from its      pack and roll the diaper from inside out so that the print is shown      outside after  you roll the diaper. Put a small rubber band around this      diaper roll so that the roll is intact. Repeat this step until you finish      rolling all the diapers. Below is how you are expecting to see. Note: make      sure to put the rubber band slightly above the      center.
 2. After you roll all the diapers,      take the big feeding bottle and 
put a big rubber/elastic band around      the bottle. Now, gently insert 1 diaper inside the elastic band in      such a way that that the design shows outside. Here is how:
 3. Start inserting the diapers      until you make 1 full round around the bottle. Now, the diapers and the      bottle are firmly bound together. I used up 7 diapers for this first      round.
 4. Now, gently wrap another big      elastic band around the first set in step 3. Start inserting diapers one      by one into this new elastic band. Once you are done, your second round is      finished. Here is the picture.
 5. Now, for the final round in      tire one, you will need to wrap the rolls with 1/4 inch lace. This      is to hide the rubber band which shows in the outer layer. Cut a      small piece of 1/4 inch lace and wrap around the band. Here is how it      looks:
 6. Now, insert another big      elastic band around the 2 tires. Repeat step 4. Here is how the final      version of tire 1 would look like with 3 rounds of diapers. Note: Don’t      worry about the big band showing for now.
7. Now the second tire begins.      Insert a big band around the feeding bottle, above the first tire. Repeat      step 3 and 5. Note: I forgot to put lace for the second round. You can      lace the second set before you insert. Here is how it looks like. This      completes tire 2.
 8. For the final tire, you might      have to attach a small feeding bottle to the big one. Use a rubber band to      tie the big bottle to the small one. I removed the nipple for the small      one and turned it upside down so that its easier to tie both the bottles      together.
9. Now, repeat step 5.
 10. The 3 tire cake is now      complete. But you will still see 3 big bands around the tires. This is      where you will put your 1 inch width lace to cover up. Just tie the lace      around the 3 bands. I used light colors which are not showing in the      picture. You could go with a darker color.
11. The final part of the cake is      mounting the teddy bear on top. I took the remaining lace and gently tied      the teddy's legs to the feeding bottle. This made sure that the bottle is      hidden and the teddy is not falling about.
12. As a final step, I wrapped      the remaining lace around the teddy! At this point you can also decorate      this with all the optional items I mentioned (I however did not for the      reason I mentioned above) .Here is the final version of how its looking      like:  Note: You can keep a plate or any base to hold everything      nicely in the bottom.
 13. Below is a version which has      the optional items mentioned. This is not the picture I have taken. I took      this picture from external source to just give you an idea of what it      looks like with gift items on the cake.
   
So, next time when you are invited for a baby shower, surprise the expectant mother with this! She will forever remember you for your thoughtful gift. I hope you all liked this Diaper Cake.
Edit:
1. Please warn the mother that the diapers are no longer sterile as they are removed from its packing.So, she will have to avoid using the same diapers for her baby. This is mostly for a keep-sake-gift. 
2.  I posted this in another ladies forum and they loved this!