Text summarization is a fairly complicated topic. If you have a need to do this in a serious way, you may wish to look at projects like Lemur (http://www.lemurproject.org/).
However, what I suspect you really want is a text abstract here. If you know what part of the document contains the body text, locate it using an HTML parsing library like BeautifulSoup, and then strip out the HTML; take the first sentence, or first N characters (which ever suits best), and use that. Sort of a poor cousin's abstract-generator :-)
