I've mapped my capslock key to control using the Modifier Keys mapping in System Preferences: Keyboard. I've also tried mapping to "right control" instead of "left control" as per http://hints.macworld.com/article.php?story=20060825072451882 using a plist editor.

The mapping seems to work in all cases except one: I can't use capslock with left-shift to make key mappings or apparently do anything else. capslock (as control) with right-shift works. I'm primarily testing by using control-tab / control-shift-tab to switch between tabs. Using the on-screen-keyboard viewer, I can get capslock-shift-(just about anything) to work, but not capslock-leftshift-tab.

My best guess is that somehow the particular keyboard I'm working on is faulty, but I'm curious whether anyone else can reproduce this or has any ideas.