It looks to me like it's just towers that are tall enough to be registered with the FAA. I checked its coverage of the area where I live, and it had none of the towers that are mounted on buildings and water towers and missed a lot of the freestanding towers that are short enough not to need lights. On the other hand, it also includes towers for public service and broadcast radio that don't have any cell antennas.
It's better than nothing, but don't take its contents too seriously.
In the particular case of Fort Bragg, it shows a couple of towers for Edge Wireless, which the gsmworld site says does GSM 1900, confirming the info that Cingular works there. There's also a note on the cellreception site claiming that T-Mobile works, but since T-Mobile claims no coverage in that county at all, I'd guess they were roaming onto Cingular.
R's,
John