2013's new SimCity will be the first in the franchise to support online multiplayer. As we described from EA's announcement event: "Decisions you make in your city will affect other players' cities--especially as the virtual world will have to deal with the repercussions of limited natural resources."
Multiplayer is just one aspect of SimCity's online functionality. The game is being "built from the groud up to support online," Maxis' Andrew Willmott said at a GDC panel.
One of the goals of SimCity is to make it a game you can "play anywhere." In order to do that, it relies on cloud storage. Because the game save is in the cloud, players will have a continuous save file that they can access from multiple devices--such as (but not limited to) an internet browser. Games like SimCity are "data-driven," meaning it will only require "small upload bandwidth" to have a continuous gaming experience.
And while online is a key feature for SimCity, Maxis has implemented an asynchronous server model, so that there's "no reliance on a dedicated live server" in order to continue playing the game. That's a philosophy that Ubisoft would best learn from.