Here is a nice article on Null Object Refactoring, a process where you create a null representation of an object for use when you want to treat the object as null, rather than passing around a truely null object.

This is one of Fowlers refactoring suggestions from his book: Refactoring: Improving the Design of Existing Code and on his site: http://www.refactoring.com/catalog/introduceNullObject.html