http://hackaday.com/2016/01/25/raspberry-pi-zero-cluster-pac...
Like others, I have a stack of Pi systems doing file storage, DNS ad blocking, webservers, home control monitoring, XMBC, etc. A clean mount would be cool
You can follow along here:
Maybe when the new Compute Module comes out, someone will come up with a way to put a stack of them on the same board.
http://www.friendlyarm.com/index.php?route=product/product&p...
But,it has two problems. 1) you still need to do all the cables to connect them to the outside world. 2)Allwinner support for weird boards is often a problem. I've gotten a few of them in the "wow, look at all the features" to find that the drivers and support underneath has real problems.
The Pi / Beaglebone ecosystems have one key thing, lots of very smart people are working on the hardware/software interface level. This means that things just work. The Pi team has multiple person-decades of software improvements.