There are several areas within 5-10 miles of Apple's Cupertino HQ that have no cell service - the nearby mountains lead to valleys where I assume it's not economical to install cell towers.
I was in both Napa and Point Reyes a couple weeks back and the AT&T cell reception was pretty hit or miss. Heck, I'm about 45 miles west of Boston and the cell reception at my house is at least variable.