Are we sure it's not an overheat issue?
My 3g model would get stuck in headphone more often than not. Within 5 minutes of use, the phone kicks me off the receiver and routes the audio to the earphone jack where nothing is plugged in. I've had this problem since I first got my phone. Many times I went to the Apple Store (5 min away) and complained. Mac geniuses were adamant there was nothing wrong with the phone itself is it was most likely lint or some other debris. They gladly got out a can of compressed air and voila, my phone was normal again. Over time this became so bad I couldn't bear it. I bought a bluetooth headset so I wouldn't lose audio. When I'm on the bluetooth long enough, the phone again sends the audio to the headphone jack and I have to manually set it back to the headset.
After much trial and frustration from Mac geniuses who don't seem to be that bright, I searched and searched until I found the problem. OVER HEATING! On a great many iPhones, this is a major issue. Some don't have the problem (my wife's phone is perfect). The processor heats up enough to short out something on the board which causes the phone to think the headphones are plugged in. Cooling of the phone restores it back to a normal working capacity. A can of compressed air blows at high pressure, but it's also cool. Spraying it into the jack simply cools the board and the phone returns to normal. I urge you to try placing your phone in the freezer for two minutes when in headphone mode. It should cool the board and your phone will restore itself to normal.
Apple knows this was yet another problem with a significant number of sold units. What company wants to admit such a crippling flaw which would force a recall? Apple could probably afford the repairs to all affected units but, it's the sudden drop in AAPL stock which is what they're afraid of. A little birdie told me this is the reason Apple store employees are instructed to admit nothing is wrong with the phones in any capacity.
So for now, cool your phones and see if that takes care of it. Those of you under warranty, get to an Apple store and demand a replacement!
I was going to purchase an iPhone 4 and use the 3g as an iPod. 3g will probably be more useful as a paper weight.
