While grading homework assignments turned in as IPython notebooks, I wanted to insert my remarks as highlighted text in a Markdown cell. You can use the following CSS style and markup to do it. In fact,

ipynb-highlights

In a code cell at the beginning of your notebook, enter:

%%html
<style type="text/css">
  span.ecb { background: yellow; }
</style>


Then in any other markdown cell you can put:

<span class="ecb">Comments by ECB are highlighted like so.</span>

and they will turn yellow.

Updated 8 May 2014 to reflect new security model in IPython 2.0, as detailed here: http://ipython.org/ipython-doc/dev/notebook/security.html