I have exactly the same issue with my MBP (2 weeks old), tried my old headphones, which I had been using for 3 years on my old G4 Power Book, and the speakers kept cutting in rather than the audio routing through the headphones, the slightest touch would either significantly degrade the audio via the headphones, or worse still cut the MBP speakers in (leading to one very annoyed other half as she kept getting random blasts of Battlestar Galactica very late at night! and also many annoyed train passengers on my journey to work as my music cuts across the train carriage should I dare to move my head 2 degrees to the left, or the train 'bump' the connection), this effectivley renders the headphone socket unuseable and significantly impacts the MBP's effectiveness as an efficient and portable media centre.
I see from the Apple insider post that this is likley caused by the design being optimized for the mini-jacks with more plastic collars than a standard stereo mini-jack, so I am forced to buy a propitiatory apple product (the headphone with built in mic, that I will never use...), having just shelled out £1.3k on new MBP this leaves a very bitter taste.
I appreciate that a full logic board recall is just not going to happen, but if Apple are as serius about Customer Service and product design as they woudl have us believe, then I believe the least they can do is provide a free set of 'compatible' headphones to those who complain.