[Joel]

We found out what's causing this crash just recently, it has to do with having Style Brush contents from version 5 copied to the clipboard. If you launch OmniGraffle 5, and click on the Style Brush's drop-down menu and clear the contents, OmniGraffle 6 will no longer crash when selecting the Style Brush (or w on the keyboard, which is the hotkey for that tool). Alternatively, logging out of your user account and back in again clears the contents.