One of the most important questions that enterprises ask us has changed from "Why APIs?" to "I know I need an API; where do I start?"
Start where you have the most amount of pain - where the business drivers are. If your pain point is in meeting demand for mobile and social apps, then start there - with an internal API strategy. If you need to innovate with partners to deliver on a backlog of business development opportunities, start with a partner API. If you need to inspire a broad community of app developers to innovate and create growth and new opportunities, start with an open API.
The lines between internal, partner, and open blur a little more every day as companies innovate at the edge of their businesses. And what starts as one flavor of API can become a different opportunity down the road. We've seen it a bunch of times - an API team learns from internal and partner projects, develops the know-how and courage to open an API to the world of innovative developers who can take the API and the business in creative and valuable directions. We've seen companies have huge success in the opposite direction - businesses starting with Open APIs and realizing that the bulk and the most valuable innovation is coming from partner and internal collaboration.
As long as you start where the business pain is - you're starting in the right place.