The Tech Report writes a modder managed to enable unlimited offline gameplay in SimCity with some quick changes to the game. On top of that, he also found a way to access the game's debug mode. Most of the debug features (including terraforming) are disabled in the retail version of the game, but some features like editing highways, even outside city boundaries, are built into the game. An offline gameplay mod for SimCity isn't available yet, but it's likely only a matter of time before it pops up online.
The RPS story notes that Kotaku managed to get the game running offline, albeit for only 19 minutes. An enterprising modder has apparently removed that time limit, enabling unlimited offline play. There are some limitations, of course. According to the modder's Reddit thread, saves and region synchronization don't work... yet. The modder was able to access a special debug mode, though, and that enables more extensive editing than is normally allowed by the game. You can see modded game in action below.