Sorry for taking all this time to answer, but I haven't been at home lastly.
I have deleted all Allegro 5.1 from my PC, I have no time right now to spend on it, I want to finish my project and I have get distracted with a lot of things lately, I don't want to add another reason.
I think what I have said is enough for the Allegro developers, if there is something wrong, they can check step by step what I'm doing in the wiki, if there, there is no problem, then I have no idea.
I think Edgar also compiled Allegro 5.1, I don't know if he was using libpng thought, I guess he was,
because he told me he didn't have any problem building it once it was in this thread ... But if didn't have any problem compiling without libpng, then obviously there is something wrong in what I'm doing. The thing is that, I have no time just right now.
Now, I'm remembering that I uploaded the binaries, what you could do, is link your project to these static libraries, and if you get the same than I, then it's a compiling problem.
Aww.. forget it... You have MinGW 4.6.1, and it's a kick in the balls now to download MinGW 4.5.2 without the installer...
Forget it man, maybe when Allegro 5.2 is released, I'll try again. I'm going to change my image to Allegro 5.0.4 again