Just picked up a '12 MBA 13" with 4GB RAM a few hours ago and had to put ML on it myself, it wasn't already updated (weird) and it runs smooth, like another poster put it, like buttah.
I could also say that my '10 MBA 13" with 2GB RAM runs quite smooth itself.
I could also say that my '10 MBA 13" with 2GB RAM runs quite smooth itself.