Steve Portigal blogs in All This ChittahChattah:
From All Things Considered the remote California town of Iowa Hill will finally get land-line phone service. And their cell coverage is spotty, at best. The woman interviewed explains that people in the community have designated areas where there is cell reception as phone booths; a nice colloquialism since we’re likely to envision a purpose-built structure rather than a warchalked wooded area. She also describes the local 911-proxy: fire your gun three times in the air and hope someone comes to your aid.
Warchalking icons are direct descendants of symbols developed by hobos during the Great Depression to indicate safe camps, friendly householders, and signs of trouble.
The term warchalking was formed by analogy with wardriving: the act of searching for wireless networks while driving in a moving vehicle equipped with a Wi-Fi-enabled computer. Wardriving in turn is derived from wardialing, a term used in the 1983 movie War Games to signify stumbling upon a connection to a military computer while looking for games and bulletin boards by means of a dial-up modem.
More on warchalking at O'Reilly Network, which is where I found the illustration.