German gaming magazine GameStar (via Spanish games site Jimdo) spoke with Maxis developer Lucy Bradshaw, who confirmed SimCity 5 is in development at Maxis. She also noted that work on the Spore and The Sims franchises kept the developer from revealing SimCity 5 sooner.
The magazine suggests that the new entry will run on an all-new game engine called Glassbox, and will feature multiplayer support, updatable buildings, and non-linear road patterns.
It's been way too long since we last had a proper entry in this franchise. I think I've worn SC4 down to the bone, so here's hoping SC5 is a worthy installment!