I've had the same problem for a few months now. I tried all the suggestions (air duster, Q-Tips, inserting the headphones and pulling them out several times, etc.). Nothing really works and the problem keeps coming back. The strange thing is that I don't actually use the headphone jack. My iPhone is a phone, not a music player. The other strange thing is that I have it in a Switcheasy Colors case which comes with a plug for the headphone jack to prevent lint and dust from getting in there. Yet, I still have the problem. Further, the only thing that works is resetting the phone or restoring it. Logically, if the reset or the restore do fix the problem temporarily, therefore it can't be "dust", "lint", or whatever Apple says it is. I've been using computers for over 20 years so far, and I have yet to see one that cleans out its own dust every time I restart it...
The only thing that makes a bit of sense to me is the moisture/humidity sensor being located in the headphone jack. CNN did a story on it a while back:
http://www.cnn.com/2009/TECH/04/14/smart.phones.buggy/index.html?iref=t2test_tec htues
Maybe that is what is causing my problems since the humidity here has been around 60% for a few weeks. However, my problems first appeared in winter when it is bone dry.
Anyways, I really hope that Apple doesn't turn into Applesoft in this case and does something to fix this problem soon. Enough people have it and it really is a pain in the butt. I love the phone and I simply want it to work the way it should. With any luck, this will be solved before someone gets too ****** off and decides to launch a class action law suit.