python method to extract content (excluding navigation) from an HTML page
What is meaningful and what is not, it depends on the semantic of the page. If the semantics is crappy, your code won't "guess" what is meaningful. I use readability, which you linked in the comment, and I see that on many pages I try to read it does not provide any result, not talking about a decent one.
If someone puts the content in a table, you're doomed. Try readability on a phpbb forum you'll see what I mean.
If you want to do it, go with a regexp on <p></p>, or parse the DOM.
Of course an HTML page can be parsed using any number of python parsers, but I'm surprised that there don't seem to be any public parsing scripts to extract meaningful content (excluding sidebars, navigation, etc.) from a given HTML doc. 
I'm guessing it's something like collecting DIV and P elements and then checking them for a minimum amount of text content, but I'm sure a solid implementation would include plenty of things that I haven't thought of.