The problem here is that the parent application (and the person installing it) has to know all sorts of nitty-gritty details about how the module does its business. [...] However, as I mentioned earlier, this situation has changed dramatically in Zend Framework 1.8, due to a wonderful little component called Zend_Application_Resource_Modules.

This new bit of functionality allows the modules to have their own boostrapping logic and autoloading of other modules. Thi smakes integrating someone else's module into your framework a much simpler (and easy to maintain) endeavor. One thing he notes that's missing, though, is any dependencies that the modules might have between each other.