I keep seeing this intermittent problem as well. Honestly can't remember when I first saw it, but it's been a while. I know hdiutil is used for mounting up disk images. And I do seem to notice the problem after recently working with an image(s). But I haven't been able to pin it down more specifically yet. And I've never noticed any ill effects from using "Force Quit" on it. Although I'm shell literate, I typically do it through Activity Monitor because that's where I watch my system.
Although I've occasionally run a program straight off a mounted image, I do that VERY infrequently. Much less often than I see the problem.
I've always assumed there is some problem in the unmount process of ejecting the disk images. Whenever I have noticed the CPU spike, I no longer have any images mounted.
Someone reported it as a bug to VersionTracker Pro.
http://www.versiontracker.com/php/feedback/article.php?story=20080405143610347
but I think they are really just stumbling over the root problem. Still, if they have a reproducible case, that could help debug it.
Spencer