Both CBS and Clear Channel are known for throwing up boards for their stations in areas with unsold inventory.
Out on a rural highway near me, there's 4 Clear Channel radio boards, one CBS radio board, and all the other boards owned by another outdoor firm are for lease. With the housing market in the tank, the developers aren't putting up boards for homes they can't sell out here.
The problem you have in Phoenix with billboards is they're zoned out of existence in most neighborhoods, and what's left is either along the freeway or in an older part of town, and those older parts have become ghetto fabulous.
I think there's still one board at Scottsdale Road & Thomas that's been grandfathered in. Like Zorba's, it set up shop before the city decided it was too nice for those types of things.