LinkBack looks pretty sweet:
LinkBack is an open source framework for Mac OS X that helps developers integrate content from other applications into their own. A user can paste content from any LinkBack-enabled application into another and reopen that content later for editing with just a double-click. Changes will automatically appear in the original document again when you save.
Looks like it goes about 90% of the way towards the convenience of editable embedded objects, without all the problems associated with that last 10% of trying to get everything to actually be edited in a window within the embedded document. It’s also interesting that this is an open-source project, spearheaded by 3rd-party software developers Nisus, OmniGroup and Blacksmith rather than by Apple itself.
LinkBack is currently being integrated into Nisus Writer Express, OmniGraffle, OmniOutliner, ChartSmith, Stone Create, Border and a plug-in has just been released to paste LinkBack data into Keynote 2.
(Thanks to Nivi for the link!)