The September meeting of the Oro Valley Amateur Radio Club has a nice presentation by Bill Hickey, AB7AA, on a pretty cool propagation prediction program.
VOACAP stands for "Voice of America Coverage Analysis Program". It's a software program that allows hams to input certain relevant data (such as antenna type, height above terrain, and power levels), and produce a reasonably accurate model showing where signals will reach.
Propagation predictions are a bit like weather forecasts - they are often based on good science but there are too many variables involved to be 100% perfect. Still, this looks like an interesting tool and I'm planning to play with it a bit.