in the days before ebay, it seemed online auctions were a very good idea that would only get bigger. want to start a dot com today? federated logins i think is a very good idea that will only get bigger.
there is already a standards movement, The Liberty Alliance, and an open source implementation, Source ID. I dont know why yahoo, hotmail, etc are not pimping out login services right now. They should be providing a simple API to allow people to use yahoo for identity management.
there will be a few big, public, fairly low-trust federated identity services, and there will also be lots of more localized identity services that are much more trusted for a given group. personal telco for instance needs a trusted authentication system and an up to date person database.
there are so many websites, so many logins. a single signon system that is simple and trusted is gonna be big. possibly a local application as important as the web browser will always be running, requesting and providing credentials of all sorts, informing the user of who is asking for what, managing crypto keys and certs, etc.