Hi again.
I've been messing arround the problem and it turned for me to be a little bug in Chameleon (I use the last version from Chameleon web site).
I set the GUI off to see what was taking Chameleon as hd(?,?) for each menu entry, and discovered this:
WINXPMC hd(0,1)
MACOSX hd(1,2)
MACOSIDE hd(2,2)
MACOSX hd(4,2)
I have to clarify that MACOSIDE is another Snow Leopard installation that I have in an IDE drive (just in case).
As you can see, MACOSX is listed twice and it starts whatever of the two options I choose, but I do not have the fourth HDD (also it is numbered not as 3 but 4, what is even more strange).
Finally I discovered that it was a kind of misunderstanding by Chameleon of what my BIOS was detecting, because I had selected the CDROM as my first boot device (I have an IDE DVD reader and a SATA burner). So I decided to change my first boot device to the SATA burner, and voila! the second MACOSX menu entry is gone.
I hope that this experience can help to anyone having the same problem, and also to the Chameleon designers, to find out the reason of this strange behaviour.
Thanks.