I had never noticed the __path__ attribute that gets defined on some of my packages before today.  According to the documentation:

Packages support one more special
  attribute, __path__. This is
  initialized to be a list containing
  the name of the directory holding the
  package’s __init__.py before the code
  in that file is executed. This
  variable can be modified; doing so
  affects future searches for modules
  and subpackages contained in the
  package.
While this feature is not often
  needed, it can be used to extend the
  set of modules found in a package.

Could somebody explain to me what exactly this means and why I would ever want to use it?
If you change __path__, you can force the interpreter to look in a different directory for modules belonging to that package. 
This would allow you to, e.g., load different versions of the same module based on runtime conditions. You might do this if you wanted to use different implementations of the same functionality on different platforms.
What is __path__ useful for?