If they do make it, would anyone have any idea if we would be able to mod the landscapes, item models ect, into skyrim?

Use the official thread, por favor.

http://www.gamesas.com/topic/1358980-official-tes-multiplayerco-op-thread/page__pid__20494298#entry20494298

In all liklihood... no.

Customization like a CK editor would be strictly forbidden, as it would create unwanted instability for the whole MMO. Think about how many issues in previous ES titles since Morrowind that can be attributed to "It's your mods, dude!"

Mod the MMO stuff into Skyrim? I see no purpose for doing such a thing.

I think it's important to keep them separate (except the lore that will be shared and most likely made canon) because they're made by different studios and I wouldn't like the mmo to swallow the single player series when it grows big.

The landscapes in the MMO can't be as beautiful as they are in TES, remember the game has been a long time in production, it has to scale for a lot of weak computers if they want to sell big, the game could look ugly unless they pull an Age of Conan 2 and many gamers can't run it.

Use that thread please.

No, you cannot use the assets from one game and move them to another.