I\'m developing a Windows Phone 7 app but don\'t want to re-implement my own identity / authentication service - espcially when we know the user will be signed in with a Win
It's true that the Windows Live ID will not be available to apps. Presumably to protect consumers privacy.
There is however an Anonymous Live Id available now as Istvan mentioned would be the case in the talk referenced in your link.
This is accessible via the ANID here for your reference.
UserExtendedProperties.GetValue Method (Microsoft.Phone.Info)
To what extent you can use this for login services will depend on your requirements. There are many different ways services can interact with users to identify service features that are only relevant to that user.
I haven't looked extensively at messenger connect that Indy mentions.. it looks interesting. Nick Randolph posted a walkthrough of some of what could be accomplished early on here if you'd like to look into that combined with Indy's info.
Windows Phone 7 Beta with Windows Live Messenger Connect
Have you looked Windows Live Messenger Connect? It recently came out of Beta (announcement). Windows Live Messenger Connector provides Javascript and REST APIs for applications to use the framework, here is the starting point for the documentation. You can use the Windows Live REST explorer to explore the REST API, there is a sample desktop application that shows how to integrate Windows Live Messenger Connect.
HTH, indyfromoz