You could use screenlock 1.0. Just stick it on your dock and click it when you want to hide what you are doing. It will display the login screen but everything you had running is still running and is visible once you log back in.
Go to System Preferences, Keyboard & Mouse, Keyboard Shortcuts. Click on the "+" button. Add whatever shortcut you want for the "Sleep" item in the Apple menu.
Other than sleeping the machine, I find Ctrl+Shift+Eject more useful. That just sleeps your display. Also it's much more appreciated with no-damn-button-to-shut-down-only-the-display designed iMacs.