LinkBack
LinkBack is a way of embedding content between applications in such a way that you can always go back and edit the content with the software that created it.
For lots of information about LinkBack, and a list of applications that support it, check the web site at www.linkbackproject.org.
To embed OmniGraffle content in another LinkBack application, just copy and paste the content like you would normally. The same goes for embedding other applications' content in OmniGraffle diagrams.
To edit OmniGraffle content embedded in another application:
Double-click the OmniGraffle content in the other application's document; an OmniGraffle window appears containing the content.
Edit the content in the OmniGraffle window, just like you would edit a normal diagram.
Select Save from OmniGraffle's File menu; the content in the other application updates automatically.
To edit other LinkBack applications' content embedded in an OmniGraffle diagram, just double-click it. When you save in the other application, the content in the OmniGraffle diagram updates.