Ronn Kling's [Object Oriented Programming with IDL](http://www.amazon.com/Object-Oriented-Programming-Ronn-Kling/dp/096712705X/ref=sr_1_3?ie=UTF8&s=books&qid=1281471500&sr=8-3) is an excellent introduction to object-oriented programming with IDL. The beginning chapters introduce object-oriented concepts like encapsulation, inheritance, and polymorphism along with IDL's basic syntax for creating classes. This should be straight-forward for anyone comfortable with writing normal routines in IDL.
Later chapters cover more sophisticated topics like features added in IDL 8.0 like object overloading, garbage collection, and the new syntax for calling methods and instantiating an object. There is also a chapter and appendix dedicated to showing tricks for getting around the lack of object-oriented features in IDL.
Overall, this is a great way to break the ice into object-oriented programming if you have been reluctant. Kling not only gives the basic syntax for IDL, but gives a nice introduction to object-oriented programming itself, as well as a discussion of situations when it can be most useful.