In a post this past Sunday Om Malik suggested that user's want aggregation not federation. While I totally agree that user's want aggregation (e.g. having all their relevant information in one place) I don't believe aggregation is in conflict with federation. Rather the two concepts are orthogonal.
I associate aggregation with API access to my data distributed across the web. The exception is closed networks like Facebook that provide all the services within a walled garden environment. So for aggregation to work in the "open web", it must be able to access my data whereever I've chosen to place it. This requires explicit user consent (ala OAuth) for the aggregator to access my personal data at different services.
Now in order for me to grant consent, and for the aggregator to be able to access my personal information, I need to authenticate to the service provider of my data. This authentication step is simplified by using federation (e.g. an OpenID valid at all my different service providers).
So federation really enables a safer, more secure, aggregation capability for users.