I've never cancelled a trip . . . but I have had to abort a couple of times. Once was due to remnants of a hurricane making it to the Finger Lakes region of New York back in 2006. It turned creeks into rivers and we were getting firehosed by sideways rain. We hightailed it back to the car via the interstate (we were only one exit from the car) and a state trooper informed us that it was illegal to walk on the interstate and asked "don't you guys watch the Weather Channel?"
The other instance was last year, halfway through a 5-day, 85-mile hike on the hottest day of the year. My hiking buddy and I had laid out an ambitious agenda and broke down from heat exhaustion and severe blisters - my boots/feet got wet and never dried out, so the skin was just sloughing off by the fourth day. We bribed a nice couple into a ride back to the car.
Besides that, a backpacking trip doesn't seem to be official without some kind of precipitation.