This worked for me on Ubuntu 12.04:

First, add a ppa that has old and new versions of Python, and use that to install Python3.3:

    sudo apt-get install python-software-properties
    sudo add-apt-repository ppa:fkrull/deadsnakes
    sudo apt-get update
    sudo apt-get install python3.3

Now, install the correct versions of ipython, ipython-notebook, ipython3, and ipython3-notebook:

    sudo apt-get install ipython=0.13.2-1~ubuntu12.04.1
    sudo apt-get install ipython-notebook=0.13.2-1~ubuntu12.04.1
    sudo apt-get install ipython3=0.13.2-1~ubuntu12.04.1
    sudo apt-get install ipython3-notebook

Now, the command 'ipython notebook' should run your system's default version of Python. The command 'ipython3 notebook' should run Python3.3.

I test the current version of Python with:

    print(3/2)

If you're running Python 2, you'll see 1. If you're running Python 3, you'll see 1.5.

Hope that helps, and happy to help troubleshoot if it doesn't. I'll add this to the readme on the project page [1] when I get the chance.

[1] https://github.com/ehmatthes/intro_programming