iZooLu has announced Lock Desktop 1.0 (http://www.izoolu.com/) for Mac OS X (10.6.6 or higher) on the Mac App Store in the Productivity category. The US$0.99 app will secure your Mac and return you to a log-in screen.
It allows you to switch between users from the log-in screen as applications and data continue to run on the other desktops. Once locked, you also have access to the log-in, restart and shutdown buttons.
Additionally, Lock Desktop allows you to switch between users from the log-in screen. Applications and data continue to run in the background.