[Joel]

It's read-only probably due to running Graffle 4 without a license, in demo mode.

If your copy of OmniGraffle is licensed, all you need do is open up the Graffle files 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,