It seems that smalltalk-like systems, and derivatives such as the lively kernel contain clues as to what a unified meta-interface to an individual's or organisation's content might look like (and how it might behave). Integration and user adoption is a difficult problem (in general) though -- probably the best example of this is the poor (and in some circles, pretty vile) criticism that Google Wave received, though this type of feedback is probably due in part to a lack of understanding of the problem being solved.
"The content revolution hasn't happened yet!"  :-)
 with apologies to Alan Kay, https://www.youtube.com/watch?v=aYT2se94eU0