help(yaml) works for me both when I install the package directly or when I use easy_install.
In any case, if you google for last lines in the traceback, you'll notice it's not really a PyYAML issue. It appears this problem occurs when inspect.findsource() is unable to find a correct .py file corresponding to the .pyc file that is currently in use. It may happen if the .pyc file was built on a different machine or has been moved around. Do you happen to have more than one version of PyYAML installed using easy_install? If so, I would suggest you to remove all existing PyYAML installations and re-install it from scratch.
