[Joel]

Additionally, you can use LinkBack to handle this as well:

1. Make the object or objects you want to use over and over again
2. From the Edit Menu -> Copy As submenu, choose PDF

(the copied PDF now is embedded with the LinkBack information)

3. Paste wherever and however many times as desired
4. Double-click the PDF and you will get a new OmniGraffle document with the original objects in it; make desired changes, save, and it will update everywhere

Not the most optimal solution, but for the purposes of wireframing it should work well.