Kathy Harris has an interesting post on ideation and the mixture between art and science. Indeed, ideation is one of the fastest growing application types of social software. I agree that although much of the science may be developed, implementations still face the art of mobilizing the community and gaining their meaningful participation.
One of the interesting things about idea engines is they inherently address the balance of structure and emergence. Idea engines are a great example of important structure (technology) that facilitates emergence (content). The structure of idea engines does not (and certainly should not) restrict the emergence of a wide variety of ideas. See Spigit and Salesforce Ideas as examples of idea engine structured social offerings.