I agree Nick - it is a concern especially on such a basic function (sound) - Linc is right, the issue is microswitch, sounds like a bad design or a huge batch of sub quality switches, checking the forums this issue is common on MBP's.
Judging by the date and lack of further activity I assume you managed to get yours fixed at the genius bar ?
I finally fixed mine myself but posting reply for others who have same issue as this - however buyer beware folks, try this at your own risk !!!
The fix won't be liked by Apple Support, the genius bar, or the repair shops but it worked for me after over 12 months of using headphones / external speakers - WHAT A PIA !
Held Macbookpro on it's side with screen open and headphone/ audio port facing toward ground.
Took WD40 can with straw dispenser and placed straw inside audio port - sprayed 3 or 4 one second burst of WD40 into port.
Held MBP that way for 30 seconds giving MBP a shake to drain WD40 out
(I performed this whole excercise outside where the WD40 could drain out without an issue ;-)
I then took an old set of headphones and re-inserted / removed them from the audio port approx 20 times in and out with music playing on the MBP - intially when inserted music will only come from headphones then eventually MBP internal speakers start working !
I've tried everything else including the cocktail stick and bent pin inserted inside the port to try and free micro switch - nothing worked it was stuck fast.
As stated, can't endorse this for anybody else - you do so at your own risk - just stating this is what I did to get mine working again !
Well when I brought it in to the Apple Store here in Columbus they knew exactly what it was and replaced it for me. So for Apple at least it is a known issue. I did shell out for AppleCare after that incident. Since it's a year since my original post I am happy to report that I have not had one issue with my MBP since. It's a great machine. When SSD prices come down some more I will double the memory and replace the HD with and SSD.
Then it should be good to go for many years to come.