Freeware Menubar Utility Provides Screen Locking with a Keystroke
Apple does provide the function users would like... just not the keyboard equivalent. If you have enabled Fast User Switching, you probably have the "Users" menu extra in your system menubar. That item has a menu selection called "Login Window...", which will get you there. Now, there are several ways to get a keyboard shortcut attached to that item. One is to use Quicksilver, which is what I did.
Quicksilver comes with a built-in script called "Fast Logout," and it's basically the same as a call to the Mac OS X login screen. To get my Ctrl-L shortcut, I just created a trigger for it in Quicksilver.
Another easy way is to get rid of Apple's "Users" menu extra and install the free, open-source menu extra WinSwitch instead. Open WinSwitch's preferences and you'll discover you can set a keyboard shortcut to open the login window!
Still, there's always room for a better idea if you're a Mac user, so I was delighted to discover Utility Lock today. I haven't tried it yet, but from the looks of it, Utility Lock is a specialized tool that goes to the heart of what users want in screen locking. It sits in your menubar and provides a quick means to lock up your screen at a keystroke. It doesn't use the Fast User Switching function at all, but instead is more like DeskShade, which has a screen-locking mechanism that puts a big padlock on your screen. (But unlike Utility Lock, DeskShade ain't free.)
Version as tested: 1.0.1.