Is there a keystroke that can lock the Mac OS X screensaver?

You can create different methods to quickly lock your screen on OS X. There is a comprehensive article here: http://www.macworld.com/article/49080/2006/01/lockscreen.html

One simple way is to:

Open Keychain Access -> preferences -> tick Show Status in Menu Bar

When you click the little Lock icon in your status bar the first entry is Lock Screen. You should be able to bind a hotkey to this in System preferences -> Keyboard shortcuts.

Control-shift-eject will blank the screen. If you wait until.. mmm.. sometime later this year... you may find that it also locks the screen...

Snow Leopard provides a way to perform a Fast User Switching Lock Screen command from the keyboard using:

• Automator to create a service that calls:

/System/Library/CoreServices/Menu\ Extras/User.menu/Contents/Resources/CGSession -suspend

• Create a system wide keyboard command which runs this service. I'm using Control-Option-Command-L.

This is all documented in this Art Of Geek article.