Fabien Potencier has posted the third and fourth parts of his "Build a framework on top of Symfony2 components series to his blog:

• Part three adds on another page to the sample site, creating a front controller and changing the output to use "setContent()" instead of just echoing the data.
• In part four he refactors the code to be a bit more readable, adds in the Symfony2 Routing component to correctly get the requests to the right controller and an example of how to generate routes based on route definitions.

You can find the other parts of the series here: part one, part two.