I'm liking the Google Browser Sync extension for Firefox.
Its doing a nice job of keeping my bookmarks (but not other browser settings) synchronized across two laptops. I had previously been experimenting with Foldershare for syncing the "bookmarks.html" file.
I can see this model of server-mediated sycnchronization being useful for some sorts of identity info. Everytime I start up the browser, it would go out to the IDP server, retrieve updated identity and then cache it for (possible) later use at SPs.
If not ruled out by the nature of the identity, or the need for authoritative data, the IDP wouldn't necessarily know if and when the browser subsequently presented the identity to SPs.
As clients get smarter, devices and 3rd party identity provider can work together on the user's behalf in these sorts of interactions. Grossly oversimplifying, its enabling this style of coperation that is the focus of the next phase of Liberty's architecture.