Brian Kramp today announced the availablitiy of the Windows Live ID SDKs in its alpha version. The Windows Live ID SDK basically means that you can add the security and ease of Windows Live into your own applications. If you have a program that requires the user to log in, you can now use Windows Live ID to do so.
To download the SDK, go to https://connect.microsoft.com/site/sitehome.aspx?SiteID=347 and click on the download now link. You will then need to sign in using your Windows Live ID (if you have never used Connect, you might need to quickly register too). So what does the SDK include? There are three main things that are included, the documentation for the SDK, including all the API calls etc, then there is a sample application that demonstrates how this can all be applied, and finally the source code for the sample application, so you can see how it works and apply that to your own programs.
As you can see, the sign in interface for this application looks remarkably like the Live Messenger sign in screen, and uses the same data store for the email addresses it shows, complete with display picture.
There could be great scope for this in third party applications, and will be interesting to see what companies adapt this into their programs.
Update: The SDK has just been announced on the Windows Live ID Team blog here. In which it also gives the link for the MSDN forums that you can go to with queries/feedback regarding the SDK. Still nothing from http://dev.live.com though.