I've looked around but can't seem to find any good answers. Would appreciate any pointers.
I don't believe there is a built-in, system wide Keyboard Shortcut. The quickest way to activating the screensaver via the keyboard is either via a "Start Screensaver" command using a launching applications such as Quicksilver, Alfred, or Launchbar.
The quickest way of starting the screensaver using your mouse is via Hot Corners. Open System Preferences, click "Desktop & Screen Saver", click over to the "Screen Saver" tab and click the "Hot Corners" button in the bottom left. There you can specify a corner to roll over in order to start the screensaver.
If you want to start the screensaver quickly, because you want an easy way to lock your computer, a possible answer is located in the high-view "Mac OS X Hidden Features Tips & Tricks" question that remains active.
shift + control + eject send display only to sleep (great for locking your computer instantly)
Use Automator. Simply open automator and drag in the "start screen saver" action, select "no input" under the "services received" menu, and hit file > save as "Start Screen Saver". This will now appear under System preferences > Keyboard > Shortcuts > Services > General, where you can assign it any hotkey you desire, just as you would any other system action.
Many moons later, I wanted the same thing and I found a solution. I use BetterTouchTool to manage all of my keyboard/mouse/trackpad shortcuts and configurations. So I created a keyboard shortcut that runs a terminal command when pressed. To launch the screen saver from the terminal, use this command:
open -a /System/Library/Frameworks/ScreenSaver.framework/Versions/A/Resources/ScreenSaverEngine.app
Now you can launch the screen saver with any keyboard shortcut (I use F19).
I agree with Jason's answer - the only built in way to lock the screen (which sleeps the display but does not engage the screensaver animation) is the shiftcontrolmedia eject shortcut.
A free solution would be to use RedSweater's product FastScripts to map your choice of keyboard shortcut to call one of the AppleScripts that are floating around the net to toggle the screen saver engine rather than just let the display sleep. I'd try making a simpler script just to open the engine at the hard coded path if you only wanted to engage the saver.
I am a very happy user of LaunchBar which allows a quick keyboard only method to start the Screen Saver. It's not really a keyboard shortcut in the strictest sense (first you activate Launchbar with it's own shortcut - then key in sss or whatever you chose/taught lanchbar to use for the built in Start Screen Saver action.) Any other commercial "launcher" product should work as well to activate the ScreenSaverEngine app that is hidden deep inside /System/Library/Frameworks/ScreenSaver.framework/......
Here's a bit of a hack involving the Keychain Access utility that allows you to define a shortcut to lock the screen -- it's five years old and I haven't been able to verify if it still works, though.