I think I got it to work fine now. Everything depends on the kind of moviment you do to raise your phone to your ear. If you have your arm straight down and move the phone to your ear it does not work. It works with shorter moviments. I hold the phone in front of me as if I am reading the screen with my arm bended in 90 degrees. I bring the phone to the ear and it works like a sharm every time.
How did I founf that out? I went to the Apple store and I was able to see that everybody was having dificulty with using the feature. We tried several phones and it seemed to work randomly. After spending a lot of time behaving like an idiot moving the phone up and down thousands of times I sat down and it started to work much better. That was when I realized that it was working better because I was not moving the phone from all the way down but from much closer to my chest.
Now some important information. First, the raise to speak feature works from any screen that is on, even from active application screens. For example, when reading a message, I raised the phone to my ear and it worked fine. At the apple store, some of the apple advisors thought that it would only work if you were already in the Siri screen, which is not true.
Second, I have to say that I actually got my phone replaced just in case it was a hardware issue. However, I tried the new phone still using the long moviments and the raise to speak feature would not work. I changed to the short moviment I described before and everything worked fine. I am convinced that it is not a hardware problem.
I hope this helps. Enjoy your new Iphone 4s.