Is this only Skyrim? Or does Steam plans to do this with other games as well. Because now I'm worrying about GTA V
So far only Skyrim, but you can bet your ass Valve will attempt to monetise any and every game with a vibrant modding community that require Steam for access to their game.
Skyrim/Bethesda has clout, influence, a massive modding community and is also a good test case because it's already an old game which has couped it's development costs several times over.
There's little risk, but if people accept it, it paves the way for other studios and modders to do the same.