[Joel]

If you have OmniGraffle Professional you can export this document and turn off the read-only flag and get a working file.

If you don't have Pro can open up the Graffle file in question using a text editor (OmniGraffle documents are XML), perform a search for the string, "ReadOnly" (minus the quotes) and change the key value from "YES" to "NO" in each case (again, minus the quotes) and save.

Hope that helps,