Please see my write up here: http://www.somethingunimportant.com/2009/03/04/get-my-patch-added-to-core/
And my ticket/patch here:
When generating routes, a nil :path_prefix is ignored, but a
blank :path_prefix ("") is treated like a string with content. In this
situation, when the strings are joined an extra slash is added to the
beginning of the path.