I was having difficulty with 3 pairs of headphones that I own. Upon insertion into the MacBook aluminum headphone jack, I would only get a very loose connection. A slight wiggle would disconnect headphone sound and revert to internal speakers.
I went to the Apple store today in Toronto, and tested a few of their units on display. Same thing

I even tested using the headphones sold at the store (BOSE noise canceling - $249.00). Same problem. Just pulling out ever so slightly, ruined the connection...
Anyone else had this problem?? I'm totally surprised there's not more about this out there...