I've been putting the finishing touches on the content for next week's Chatter for Developers webinar (did you register yet?). One of the new features in Spring 11 is the ability to work with Chatter Files via the content API. This is achieved by the introduction of a new entity in the Chatter domain model called ContentDocumentLink.
If you are familiar with the EntitySubscription entity, ContentDocumentLink works in a similar fashion. It contains a link to the Document (ContentDocumentId) and who/what the document is shared with (LinkEntityId). Documents can be shared much broader that say the following-followers relationship expressed with EntitySubscription, and as a result, LinkedEntityId can be used to identify files shared privately to you (aka peer-to-peer), a record, Chatter Group, Content Workspace, or even your entire company (by using your org id).
Cool huh. Lot's more cool tips and tricks on next weeks webinar.