I watched Sean Corfield’s presentation on duck-typing this morning, and he brought up very valid points on why we should not type parameters and returntypes.
The bigest advantage that I saw was performace. Sean said he was able to improve performance of the fusebox XML compile by 2-3 times. Joe Rinehart was in the audience and said he witnessed the same imrovement in speed for Model-Glue.
Since ColdFusion does not require types like Java does, we should take advantage of that ability. You still have to use typing when writting SOAP based Web Services and Flex marshalling.
Duck-Typing also makes unit testing easier because you can reuse your tests for multiple objects.
He mentioned that Michael and Judith Dinowitz, and Hal Helms have written articles on duck typing.
I will try to post links later in my comments.