Well now this is interesting. Hackers have already been plodding away at SimCity and found out the game can indeed be played offline. It involves putting the game into some sort of "debug mode". It's fully playable though it can't save cities or interact with other regions. By the sounds of it, the process is relatively simple. To make an offline, single player version, programmers would just have to add local saves. Something that could easily be patched in. This fully dispute's EA's claim that online functionality is so deeply interwoven into SimCity that an offline version was impossible.

Furthermore, a Maxis insider told Rock, Paper, Shotgun that the servers only handle cloud saves and communication between players. All the computational business happens on the local end of things. He stated he can't understand why EA keeps claiming otherwise.

It's also been suggested that the servers don't even react to your gameplay in real time. Kotaku found out that the game could run happily offline for 20 minutes before it realized it wasn't connected to the server. This backs up the hackers claims and proves SimCity really isn't an MMO at all, nor does it need the servers to run.

So it seems EA isn't being completely honest over their always-on functionality. The servers do enhance gameplay but are not necessary for it. Yet EA continues to state otherwise. The plot keeps thickening.

Sources:
-Destructiod
-Rock, Paper, Shotgun
-Kotaku