If you are creating a new virtualenv and want to use IPython with notebooks in this environment, it does sometimes happen, that it is not using the right python version from your current virtualenv, but from an different environment and you will get the following error:
AttributeError: ‘Unicode’ object has no attribute ‘tag’
To resolve this problem, you just have to use the following snippet instead:
python -m IPython notebook
Now, everything should be working fine.