I'm running OS X 10.7.5 "Lion". Three times now, Preview.app has gotten in a state where its icon is visible in the application switcher (⌘+Tab) but it isn't running (doesn't show up in
ps ax in the Terminal) and will not start (e.g. double-clicking a PDF document).
killall SystemUIServer, and
killall Finder and those do restart but the stuck Preview icon is still in the application switcher. I've also tried removing the saved state under Library > Containers > com.apple.Preview but that doesn't work either. The only thing that solves it is logging out and back in again.
Is there some way to get this unstuck without having to log out?