My Macbook's headphone jack stopped working, how do I fix it? my headphone jack output stopped working a couple days ago. As in - when I jack a cable (or headphones) the sound comes via the built-in speakers. 

On the installed Windows (Bootcamp) it works OK (sound coming out via headphones when jacked in). 

Any ideas how I can troubleshoot this? 

The OS is Lion, freshly updated (update after the problem first appeared).
(posted as an answer so you can accept it and mark this issue resolved)

Have you checked the Sound pref pane in System Preferences? There's a tab labeled "Output" where (I'd expect, as I don't have a portable in front of me just now) you can switch output between built-in speakers and the headphone jack. If the first toggle doesn't work, toggle it again.