Inspired by the superglue post, I got creative.
I wanted to find a way to fix it, but not make any permanent changes to the phone. Since the problem is that the jack can move a little bit in it's enclosed space, I took the back off the phone.
I don't want to make any permanent marks to the phone, like superglue could if i manage to touch anything when i'm trying to get a little bit of it in there.
So I took an old credit card, and make 2 cuts along the magnetic strip, and bent it out away from the rest of the card. Put it up against the back of the headphone jack (under the battery cover). I made it the width of the jack, and then sharpened the edge of it, so I could get it in between the phone and the plastic housing of the jack. I inserted it as far as I could, and made a mental note of how deep it would go (i don't want to lose the piece of plastic in the phone). I cut the piece of plastic (it was about 2mm long by 2.5mm wide when I was done). I wedged the piece of plastic behind the jack, to put pressure on the circuit board, then used a black sharpie to color the little bit of white plastic that was shown.
Fixed my problem as far as I can tell.
I can pull the piece of plastic out if I need to return the phone, without leaving any signs of tampering........... and it's black, so you cannot even tell that it's in there.