Hey, you hijacked my thread! ;-)
Actually, I get logged out, which results in a crash of everything in a way. As Russ said, I'll be doing something innocuous — a sizable percentage of the time it's when I've clicked the "Show Background Tasks" button — and the next thing I know, I see a blue screen, followed by the log-in fields. By that point, it has "taken down" everything I was working on, in every program I had active.
FCPX has done this every so often since installing it on this home machine 3 or 4 months ago. The home machine is an old MacPro with Snow Leopard OS.
I've been using FCPX for almost a year at work. There I'm using it on a 2011 iMac with Lion OS. There I get crashes similar to what Russ is talking about. If I'm really pushing it hard, there are days when it will crash twice in the same day. This general crashiness only started with the 10.0.6 update. I've been hoping against hope that Apple will fix this, but so far, zippo.
Yes, I do occasionally trash prefs using Preference Manager, and repairing Permissions. It may help — Who knows.