The homotopy groups of the étale topos of a scheme were defined by Artin and Mazur. Are these known for Spec Z? Certainly π_{1} is trivial because Spec Z has no unramified étale covers, but what is known about the higher homotopy groups?
Since nobody else is answering this question I will give a few vague thoughts. Caveat:
this is a completely speculative answer for various reasons, not least of which because I don't know ArtinMazur's definition of pi Since X = Spec(Z) is "simply connected", one can pretend that the Hurewicz theorem applies. I believe that H^i(X,Z/nZ) is trivial for all i, as a consequence of class field theory and the fact that Z has neither many units nor nth roots of unity. (I'm not completely sure about i = 3 and n = 2 here.)
One can then squint and imagine that the higher homotopy groups of X are trivial. This seems a little dodgy. Another direction one could go is to note that the groups H^i(X,G For what it is worth, both computations suggest that pi 


If etale pi_1 classifies obstructions to trivializing finite flat unramified Zalgebras, it would be nice if the whole etale homotopy type classified obstructions to trivializing simplicial commutative Zalgebras that were finite, flat, and unramified in a homotopy sense. I think all of these notions make sense: "finite" means that the homotopy groups vanish in high degrees and are finitely generated, "flat" means that these homotopy groups have no torsion, and "unramified" means that the cotangent complex is zero. Is that right? Presumably algebraic topologists have thought about the sphere spectrum version of this question. Are there any connective Einfinity ring spectra that are finite, flat, and unramified over the sphere? After Tyler's comments, I see that this is a bad analogy. The dictionary between etale locally constant sheaves of sets and finite flat unramified algebras (which in one direction takes an algebra and associates the sheaf of sections of its spectrum over Spec Z) just doesn't extend to a dictionary between homotopystyle locally constant sheaves and homotopystyle finite flat unramified algebras. 

