The developers took the server offline at 9 a.m. ET in order to publish Update 10. The patch, which Maxis announced in January, adds the ability to play SimCity offline and mod the game. In offline mode, game saves are stored locally on the player's computer; no internet connection is required at all. As of 10:20 a.m. ET, EA's quality assurance team is checking the implementation of Update 10 to make sure the servers and offline gameplay are working as intended. Maxis estimates the servers will be down for a total of "a few hours."
Shortly after the release of SimCity a year ago — which was marred by serious server issues that frequently rendered the game unplayable — Maxis' then-general manager, Lucy Bradshaw, told Polygon that adding an offline option wasn't impossible, but would require "a significant amount of engineering work." In addition, said Bradshaw, the studio had designed SimCity as an online-only title because it had a vision of exploring the interactions between different small cities in the same region.
Update: As of 12:58 p.m. ET, SimCity's servers are back online, and the game will prompt players to download Update 10 the next time they sign in. You can watch a brief tutorial for offline mode below; more details are available in a blog post on the game's website.