View Single Post
You can use the selection inspector and then drag a text-style chit onto one of the items in the inspector, in order to change the font of many objects at once.

This is a strange-sounding bugó*any chance you could send a broken and a fixed version of the same file in to the OmniGraffle support address? I'm no longer on the OmniGraffle development team but I bet they'd like to have an example file to examine.