According to the source, the servers are coordinating social messages between cities in a region, as well as "cloud storage of save games, interfacing with Origin, and all of that. But for the game itself? No, they're not doing anything."
Even when the servers go down, certain cities have stayed online and playable for a period of time, suggesting off-site computation is not entangled in the main gameplay loop. The RPS source claims servers are also being used to repeatedly check for instances of hacking and cheating – and the game's verification messages to servers are creating queues and hampering responsiveness.
Finally, the source casts doubt on EA's claims that a single-player version of this SimCity would be impossible to produce. "It wouldn't take very much engineering to give you a limited single-player game without all the nifty region stuff." EA's current public reasoning about why the game needs to stay online has done little to quell frustration voiced over the game's tremendous launch issues.
Joystiq has contacted EA for comment regarding the source's claims.