How to disable to login screen shortcut (Cmd+Alt+L)
Not sure what you're trying to do, but Safari uses CMD+Alt+L for the downloads window.

Also, I used to use LockMyScreen (which doesn't work in Snow Leopard anymore), and it's shortcut is showing in my Mac as CMD+Alt+L.

I went to System Preferences > Keyboard > Keyboard Shortcuts, then looked in Application Shortcuts.