Reddit user AzzerUK claims to have enabled SimCity's debug mode and to have disabled its disconnection timer, both of which imply that there is more to the game's inner workings than EA and Maxis originally stated.
Currently, a user that loses their connection to the server during gameplay will be logged out of the game after 20 minutes. This is supposedly because the client must sync simulation data back to the server on a regular basis, in order to ease the computational load on the user's machine and to ensure the simulation as a whole runs smoothly.
Though not demonstrated in the video above, AzzerUK claims to have disabled that disconnection timer, and that playing an offline city for extended periods of time resulted in no issues with the simulation itself. Since SimCity does not support local saves in any way, it is not possible for AzzerUK to actually save anything that happens in his offline city, but the important thing is that the simulation reportedly did not come to a screeching halt after being unable to sync with the server.
The modder/hacker also claims to have enabled SimCity's debug mode, which allows for cities to be edited beyond their typically imposed borders. Though clipping and texture mapping issues are easily visible in the above clip, the traditionally impossible highways created at least appear to function properly. This supposes that, at least theoretically, the game is capable of supporting city sizes that are larger than what is currently available.