My nephew is a huge minecraft fan. He had the problem that he had spent hours creating buildings in several different worlds on his ipad, his own pc and his grandmother's pc. He wanted all his buildings transferred into the same world, preferrably onto his ipad, to show off to his friends at school etc.

I have Linux Mint 15 XFCE adition installed with no special extra stuff that I can think of.

Expecting nothing I googled around a bit and found a tool called mcedit at
https://github.com/mcedit/mcedit (Thank you Mr. Vierra!)

I easily apt-get installed git and following the guide there I were able to git clone that repository and start up mcedit with no problems. (Wow I gotta say Git is amazing! Thank you Mr. Thorvalds and Mr. Hamano! Also I gotta learn more Python because it is able to do great stuff!).

I trivially copied the worlds from his and my mother-in-laws computers into my computer by finding the .minecraft folder on both and copy-pasting the saves/<world name> folders. Using the charger cable that came with it, I connected the Ipad to my computers USB port. Two "disks" immediately popped up in the file manager. One of them had a minecraft folder. I could copy the world from there albeit it didnt looked exactly the same, only had some of the files that the pc versions had.

I then could open the different worlds in mcedit and easily copy and paste all of it into one world. Pretty good and easy to understand user interface.

Transferring it to the computers went fine, and my nephew could enjoy every one of his buildings in one, big world that had stuff from the latest minecraft update in them.

It worked partly on the Ipad as well but all chests and buttons and other stuff got turned into wool there. Not sure why. So he couldnt get all the stuff from his chests and advanced stuff like portals and train tracks and levers didnt work 100% - but he could still show off his buildings. (I tried playing around a bit to make it work, but copying all the files somehow turned the world into a "Creative" world where you dont have chests, and copying only the large .dat file made it remain a "Survival" type world but still caused everything unfamiliar to it to be wool and chests to be gone.)

I still think this is a great success story and I managed to bring much joy to one kid and impress my sister-in-law and my wife.

Congratulations (and yeah, Git is way cool). I have used mcedit before but it has been a over a year ago. Do you know if it requires that all worlds be built from the same version Minecraft? I will google for this but just happened to see your thread and it reignited some interest.

Also, at least in earlier versions, Creative worlds did have chests. In some collaborative worlds, I have a few rooms with as many as 12 chests and 8 forges

Sorry for really late reply. The version that the different worlds were built does not seem to affect anything.
One of the things I did was to copy from my oldest beta minecraft world.