I'm looking for some pointers on how to reproduce the Apple Expose effect in my own app. Is there an API for Expose? If not how can it be replicated from scratch?
No, there is no official Exposé API.
What effects are you trying to achieve? This old Apple Mailing List thread is on the same topic. The thread includes some info on how to get a window scaling effect similar to Exposé. I'm also guessing that your other question about dimming the desktop is related. Let us know what else you'd like to do and maybe we can offer some advice.