LinkedIn, naturally, had a blog post with an introductory video that explains the platform.
While Facebook obviously has an enormous lead in terms of developers, it will be interesting to see what traction Linked does or does not gain. They indicate they will be supporting OpenSocial, which will allow developers to make apps that run in other OpenSocial-compliant social networks as well. They also have a strong user base within the business/professional community (include me) which could be quite attractive to developers as well.
In any event, I look forward to watching the announcements from LinkedIn as they roll out this developer program over the next few months.