So, finally, I decided to get the answer on my own hands, and to publish it if I had succeeded -which is the case- so, sorry if my english is not that good or it is not well written (I speak spanish as my first language). I´m just trying to make a little contribution.
So, I got to investigate a little more on the phone. I got to this very helpful site: http://www.ifixit.com/Guide/iPhone/ (thank you guys for that guide - awesome work-). There I figured out how to where the real problem is: the headset jack connector. And, more specific, and how I get to know later on: the moving parts on the headset. (there are 2 moving cooper contacts inside; the first one is in the very first part of the jack, it´s the first cooper contact that you see from the outside, and it´s located to the side of the volume buttons -that cooper contact is in fact, a button that tells the phone to play or stop playing the ipod-. The other contact, which is our interest now, is at the end of the jack, and is the one that tells the phone that there is a headset plugged).
I imagined it had to be a contact inside, because when I blew with my mouth inside, sometimes it recovered its original state. But in a couple of minutes, the problem came back. So, I noticed that it was the moist in the air from my mouth that was making contact inside, and when it got dry again, it would get problematic once more.
Well, to the solution: I wont be showing here how to open the phone, I believe there are hundreds of tuts and videos and sites telling you how to do that. What I´m going to describe and show here is how to get resolved the real contact that makes this problem to happen and how to fix it. It is pretty simple after all. It is a little time consuming, but after all, the phone is worth the try. You won´t break anything or loose anything at all. It´s just a matter of cleaning inside your phone in a very specific place.
It´s also important to notice that I have a 4GB first generation phone, which I have almost have for a year now, in Chile, with no warranty or Genius bar at apple stores, or even something close to that. I have my phone under ENTEL PCS network and running 2.0.2 now, jailbreaked and unlocked with winpwn and itunes as I already told you.
Well, so let´s get to the point; after you have opened your iphone, you´ll notice that the headset jack is attached to the rear cover. In many guides they´ll tell you to disconnect it, but in this case, we won´t. That´s because you need the jack to be working and displaying on screen if the headset is plugged or not. So, just keep it open, but attached.
As you may notice, there´s a little, very tiny piece of transparent plastic covering the very end of the jack. That piece of plastic, is the moisture "sensor", which will get red if it gets wet. (mine was red from under -inside the phone- but still white on the side to the jack -the color yu´ll see if you look through the jack itself, on the bottom of it-). The color of it at this point don´t mean anything since you are the one opening it and knowing if it´s ok or not for you. Well, that piece of plastic has to be removed, it´s just a cover, it has no cables or contacts attached to it, it has no electronic function at all... it´s just there for you to look at it from the outside.
So, after you have removed the plastic cover, you´ll see the real contacts, and of course: dust, losts of it... fuzz and dust, and even something else, which I´m pretty sure you´ll find, because you haven´t solved your problem yet at this point and I´m pretty sure you´ll find something solid between the contacts.
So, you can see from very close, that the end of the jack has a squared cooper contact all around it. And, on the right side, there´s another cooper contact. If you get your ear-set plugged, you´ll notice how the second one moves away from the square one. and, between them, sure there must be fuzz, dirt, dust, hair, could be anything, but it is blocking the second contact to return and touch the square frame... that is the very reason why you can´t get it to work like it should. So, find yourself a nice tiny brush (like the ones for makeup), and clean that spot. But to do so, you´ll have to get your headset plugged in, so the two contacts are separated from each other and the spot gets free to be cleaned. (it is very reliving to do so by having music playing, because, when you release it, music will be coming out from the speaker as it should have).
And, well, that´s pretty much it. You have solved your problem. I even get to the point where I was wondering if I had to replace the headset jack with a new one (because they´re very cheap: http://www.ifixit.com/iPhone-Parts ), but that was just if I had had any big problem with the operation.
Well, as you all are grown ups (but maybe there are some that don´t feel much like that), I suppose I don´t have to repeat that this procedure and any other that involves the opening of your phone, could damage it for ever. So do it under your own risk. (but in my opinion... is not that risky, and not that difficult to do it... just be careful and think of it as your own son)...
This is my phone, finally in good shape again !!!
I hope this helps you out.
Any comments would be appreciated.