Todd says there are thirteen Guardian Stones out there; and we have confirmation that the Guardian Stones are matching up to the traditional TES birthsigns (of which there are also thirteen).

But what about the Atronach? With the Atronach appearing as a perk under the Alteration skill tree, should we assume that the Atronach guardian stone has been replaced? The perk fulfills it's typical function (regaining magicka by absorbing spells). What function does that leave for the Guardian Stone? I suppose it could provide the same function, or a similar, stacking function, but it would seem an unusual choice to have the Atronach birthsign appear in two places (as a perk and as Guardian Stone), with the same name, and the same (or similar) effects.

Is it possible that we will see a new birthsign on the thirteenth Guardian Stone? Maybe something related to the main storyline? Maybe a sign that improves shouts? The Dragon, perhaps?

Has anyone out there seen an Atronach Guardian Stone, or a stone that doesn't match an established birthsign?

There are 13 birthsign constellations, Atronach included. It's probably in, just with a variant effect.

No it isn't
Atronach Stone:

Oooh, I think I've found my Birthstone! Sure it comes with a negative, but at least this time it's much better than that stunted magicka bs!

Well, that answers that, then!

Well we can't have everything can we?
I don't know exact numbers on that one as I saw it on a loading screen on some.....uhm......not-so-official footage. It appears that descriptions on loading screen are more generalized and do not contain specific values ( from what I've seen ).

How was getting 100% total spell absorption not good?

I believe it was 50 percent, I do not think it was 100 percent.