I saw that yesterday, and there was another short thread on it. It was in the 60MB range and just said "performance improvements" in the changelog, so probably just a security patch (not that I mind a security patch, mind you!)
Nope, not out on AT&T yet, and I'm not aware of any US carrier other than Sprint who has released it. The fact that AT&T just released a security patch instead of rolling it up into the Marshmallow release does not bode well for them releasing Marshmallow really soon. But we shall see.