As Adam suggested there will likely be "avatar hosts" springing up like services such as hotmail and gmail. I register my avatar with one of them and if the sim/grid I connect to utilizes their service when I log in as my avatar.com identity I appear in the sim as my avatar. My assets (clothing, inventory, appearance, monetary balance etc) would be held on this host and will be available when I am in a sim using this system.
There's nothing to prevent someone else from registering their own though on a different service, just as there's nothing to stop anybody from registering
joshua.nightshade@yahoo.com when I'm @hotmail.com. Or I can choose to simply host my own identity server, where I would be
joshua.nightshade@joshua.nightshade.com or whatever.
But again this is still a hold-over from everyone being used to the way
SL operates and trying to extrapolate that experience to a larger system. For obvious reasons that isn't going to work in every instance.