As of 2012 we are using Git / GitHub http://www.github.com/ to manage Phing's source code.
The Phing repository is available at https://www.github.com/phingofficial/phing. Use the git tool to checkout a copy of the source code. We do what we can to ensure that at least the master branch is buildable and runable, but clearly the code in Git should not be considered ready for production.
$> git clone git://github.com/phingofficial/phing.git
Feel free to submit pull requests!