This might happen in other WordPress versions too.
I updated to editor version 2.2 through WordPress automatic updates. Unfortunately the plugin vanished from my list of WordPress plugins (error message - plugin file does not exist). Scanned my list of plugins but I can't find anything resembling any of "front-end", "front end", "frontend" or "scribu".
I tried downloading from the plugin's home page but the same issue is present. The plugin installs but there's no way to activate it because it's missing from the plugins menu.
Will revert to the last stable version. I love the plugin - use it when developing on my home server. Please fix it soon.