According to Apple Knowledgebase article TS3310, after updating to Mac OS X 10.6.3, some Macs may experience iMovie ’09 unexpected quits when scrolling through the Titles pane.  This is due to a bug with the Far Far Away title option (Star Wars style letters going off into the background).  Apple’s workaround is to avoid opening or scrolling the Titles pane in a way that reveals the Far Far Away title, or to revert to Mac OS X 10.6.2.