Sorry for the troubles you're having. Seeing that you have actually captured a crash report and posted it here, I'm going to guess that you also sent it in to Omni via the normal method.
If you haven't already, and the crasher is repeatable (it sounds like it is), please send email about it to firstname.lastname@example.org
as that will open a trouble ticket to ensure that you get some personal attention. (The automated crash reports don't necessarily tell us whether or not a crash is repeatable. Also, if there are too many crash reports for an application, the automated ones don't necessarily get looked at by a human. I'm not sure if that's a problem for any of our apps at the moment - I'm pretty sure the OmniGraffle crash reports are getting looked at, but if we don't see that it's a reproducible crasher, and how to reproduce it, we are more likely to treat it in aggregate.) Our support focus is on email - the forums are more for users looking for help from other users, and they don't go through our trouble ticket system.
I did take a look at the crash report you posted here, and I see that it is crashing in Apple's font-handling code. Is there a chance that you installed a new font recently, or uninstalled/disabled a font, or installed/uninstalled some software that might install it's own fonts? It's possible that it's a corrupt font, or that a font that we need is missing, or something like that. (Hard to know, since the crash is in Apple's code. Most if not all of the "missing font" problems no longer result in a crash, but there's always the possibility.)
It's also possible that application preferences have gotten corrupted somehow, causing a font specification that is somehow problematic, so if you can't identify a problematic font, you might try moving aside the OmniGraffle preferences (in your home folder, go to Library/Preferences and move aside com.omnigroup.OmniGrafflePro.plist, then launch OmniGraffle Pro and see if that helped). If that does fix the problem, we would be interested in seeing the old preferences file so that we can try to identify what about it was causing the problem.
Another possibility is if you installed new software recently which caused permissions on one or more font files or folders to be changed incorrectly; in that case, fixing permissions (via /Applications/Utilities/Disk Utility) may solve the problem. (I would try this as a last resort, however.)