I got mine working by following this tutorial by the letter, the modifying incrementally to my use case - http://lucjuggery.com/blog/?p=604. What was useful in this tutorial was the part where he starts the Mongo client in another container to connect to the Mongo server.
The other thing I had to do is destroy and re-create the VirtualBox machines that I created for this. Seems something gets confused if you have multiple host adapters or something. In my case I use Vagrant for other tasks, as well as a separate user account which wanted to create a duplicate host adapter. I had to go into VirtualBox and delete the duplicate entries. Then I created my machines on a specific subnet
docker-machine create -d virtualbox --virtualbox-hostonly-cidr "192.168.90.1/24" --virtualbox-boot2docker-url "file://$HOME/Downloads/boot2docker-experimental-rc4.iso" node-1