The Windows Live ID Client SDK documentation shows you quite well how to sign in silently or with the built-in dialog - yet there is a small problem if you want to use this within a Powershell cmdlet, because the threading model has to be set to Single...
If you have seen Pablo Castro's MIX Session about accessing the Windows Live Services via Atom Publishing Protocol and want to play yourself with the protocol, I've built a quick and dirty WinForms app, which logs you in with your Windows Live ID and...