At least, after hard work and many hours I managed to get Beryl + XGL working.
Now the method I followed is not orthodox, but it worked.
If I haven’t written already then I tell you that I had Kubuntu Feisty installed in my pc.
I tried to make it work with XGL but with no luck. After a lot of
frustration, and an almost damaged to death xorg (cause of the constant
changes on it’s configuration) I took the decision to switch to Ubuntu,
meaning that I would loose many of my configuration etc etc.
But that was my final solution, my last chance to get xgl (the cube) to work.
So I installed Ubuntu Feisty, by the way the installation was a piece
of cake. After that the only thing I cared about is to get XGL working.
After that my excitement went UP ! Beryl and XGL worked like a charm.
You can see some screenshots bellow: