From http://www.seattlewireless.net/archive/ezmlm.cgi?mss:6427
Line of Sight (LOS): An optical line of sight exists if an imaginary straight line can be drawn which connects the antennas on each end of a link. A radio line of sight is similar, except that radio waves will normally bend or refract through the earth's lower atmosphere, therefore the radio LOS assumes an effective earth radius of 8650 Km, which is 4/3 times the actual earth's radius. Maximum radio LOS can be calculated from:
Dmax = 4.1(SQRT(Ht) + SQRT(Hr))
where Dmax is in Km, and Ht (transmitter antenna height) and Hr (receiver antenna height) are in meters.
Example: If Ht = 30.5m (100 feet) and Hr = 15.25m (50 feet), then Dmax = 38.6Km (24 miles)
It is important to realize that microwaves don't travel like a laser beam; they effectively occupy a region of space considerably bigger than the mathematical line of sight. The entire "Fresnel zone" surrounding the sight line should be clear in order to obtain the best transmission. The Air Whidbey site (also below) includes a convenient Fresnel-zone calculator.
There exist several software packages that will let you determine LOS from DEM (Digital Elevation Model) data.
MicroDem and DemWorks Beta are free (DemWorks only until it's out of beta); ArcView costs an arm and a leg ($1,500 or so).
A good source of DEM data is the USGS 
-- beau.gunderson@overlakehospital.org
There is a free (beer) Windows application called RadioMobile that will download USGS data and calculate radio site coverage based on location, frequency, antenna height and type, and transmitter power.
-- explodinglemur (at) gmail
