Continuing from my previous post on MOS...
I didn't really mention this in the previous post, but MOS is designed to provide a hosted alternative to the server-side applications. One of the goals is to continue working with existing native clients and client access methods, so (for example) you can access your Exchange Online mailbox through OWA (running from MOS), through Outlook, or even through EAS/Windows Mobile. In order to do this, though, your client applications need to know how to talk to MOS and provide the proper credentials.
You can do this the hard way or the easy way. The hard way is running around and reconfiguring each application by hand and teaching your users how to use a separate set of credentials. The easy way is to use the MOS Sign-In tool, a little .NET 3.0 application that runs on the client desktop. It interacts with Outlook 2007 RTM/SP1, LiveMeeting 8, and IE7+.
When this application is run, it will invite the user to logon to MOS. The first time they do so, they're required to change their password. It then detects the apporpriate applications, offers to configure them to work with MOS, and then just sits quietly on the desktop, providing a seamless SSO experience.
To be continued...