How to map Caps Lock as a second Control Key on a Microsoft Natural Ergonomic Keyboard 4000?
Two months went past, and a "legitimate" solution is nowhere to be seen. Fortunately, there is a simpler and a more radical way to solve this, and all related issues — getting rid of the crappy Microsoft drivers completely. Caps Lock, and other modifier key mappings, are easily changed in the "Keyboard" preferences. Play/Pause, Volume and Mute keys are working perfectly.
All of the other media keys, and the "Zoom" slider are predictably dead, but I've never used them anyway. The "Application" or "Menu key" is mapped to ⌘ with KeyRemap4MacBook.