Skyrim is not a MMO, and I'm pretty sure Bethesda's in-house Elder Scrolls team will CONTINUE to make single player TES games.
That said... Zennimax (Parent Company) IS making an MMO.... and I have good reason to believe that it IS an Elder Scrolls title. I do have a couple of friends in Bethesda Studio that do not deny anything when I ask "How is that Elder Scrolls MMO going?"... but at the same time they don't confirm it either, because they would say "I wouldn't know... that's a whole different studio".
Read into that what you will... and yes, I could be completely wrong in my interpretation, but we'll see if my suspicions are correct when the title of the MMO Zennimax is working on, finally gets announced.