Rob Allen's Blog: View helpers in modules
I came across a situation last week where I needed to access a view helper that was in the default module's views/helpers directory when I was in another module. This came about because my layout.phtml uses a view helper that is in application/views/helpers.
An error's thrown if you try to use it normally, something about a plugin not being found. The key fix is an easy one, though - add a new Zend_View_Helper path and a helperPath for the module you want to use them in inside the application.ini file.