In an update to the State of SimCity Blog, Maxis Emeryville studio General Manager Patrick Buechner said that the company is considering an offline mode for the game that launched as an online-only, always-connected game earlier this year. I'm sure you remember how that turned out...
"Right now we have a team specifically focused on exploring the possibility of an offline mode," Maxis Emeryville General Manager Patrick Buechner wrote. "I can't make any promises on when we will have more information, but we know this is something that many of our players have been asking for. While the server connectivity issues are behind us, we would like to give our players the ability to play even if they choose not to connect."
Buechner added an acknowledgement that having an offline mode for SimCity has a lot of advantages, like providing breathing room for the modding community to experiment without interfering with online gameplay. We would add that being offline in single-player is the biggest advantage to having an offline mode because it's simply less of a hassle - especially if you have a spotty connection.
Source: The Escapist