Maxis has started a discussion on its forums about possibly allowing user-generated content in SimCity. This is something everyone was screaming for when the game came out, since previous titles were mod friendly.
No official plans for mod support have been announced yet; the developer is just trying to see if the interest is out there. To me, and a lot of the people in the Maxis forums, it's very obvious that people are interested in being able to mod the game.
The types of user-generated content and mods under consideration will be ones that "do not jeopardize the integrity of the gameplay or harm the experience of others" and don't affect "the simulation for multiplay games." Basically, Maxis doesn't want to allow any game-changing mods, and instead only wants to allow cosmetic items.