From the article: "By rewriting the game's code during "debug mode" AzzerUK turned off the game's disconnect timer so it never checked whether it was online or offline. He also fiddled with other values to almost convert it to an offline, single-player game."
The only downside, so far, is that it isn't possible to save a city except on EA's servers. For some details on how AzzerUK enabled SimCity's debug mode and made his modifications you can check out his Reddit post and for a more detailed explanation he has a post up on Pastie."
Link to Original Source