There has been a lot of talk about Facebook's new platform and how it open it is. Yes, third-party developers can now build applications using Facebook, meaning they can be embedded and distributed within the Facebook website.
While I agree that's open, it's not as big of a deal as everyone is making. Facebook isn't opening their userbase, nor are they distributing relational data to third-party services. They are still a walled garden - if you want your application to integrate with Facebook, you need to build your service on the Facebook website.
What's a better solution? Let the services directly integrate the userbase and relationship data. Make it mutual. Allow a service like eBay to relate their users using the Facebook grid... when I sign up at eBay, they can analyze my e-mail address and get my unique user ID. Repeat this process for every registered user, and you now have a related userbase within the eBay platform - not some little widget on a Facebook profile.
How else could the grid really be open? Let third-parties attach/retrieve information to/from users. If a person uses a service such as 30Boxes, let 30Boxes attach the calendar data to that person. This would allow participating calendar services to mutually display scheduling information for friends - if I'm at Google Calendar and my friend is at 30Boxes, no worries - our data is still visible to one-another.
A company like eBay could attach listings to a user so that social search engines could organize results based on how many degrees away an item is. If I went to the Facebook platform, I could search for an item and have results sorted by the grid... or Flickr could attach photos to a user, again, resulting in a good social search solution. This isn't content aggregation, so it would actually drive more traffic to these services.
Facebook doesn't see it that way. They want you to be in their grid. They don't want to share their userbase outside of the Facebook domain. That's not open.