I've got a similar problem-- the headset symbol is displayed whether the headset is plugged in or not, and the audio only works if I use the phone in speaker phone mode.
Leading up to this, I had a problem with the phone doing something weird. I use the phone to listed to music on a daily basis. A month or so back the phone started to jump to a different music file, just at random. This started to happen more and more often, and I noticed that it happened when the headphone jack got tugged or bumped. Then yesterday it started doing this on an almost constant basis, jumping from one music file to another every 10 seconds or so. So I gave up on trying to listen to music, but discovered that when I tried to make a call, I couldn't hear anything unless I put it in speakerphone mode.
I'm guessing that this is a hardware issue, and that it would be cheaper to replace the phone than to try to fix it, but just in case...
Thanks