Just read an interesting article - http://www.runningtimes.com/issues/99janfeb/hrm.htm - with regards to this:
Three studies from an international conference on the use of heart rate monitors, recently published in the Journal of Sports Sciences, reported that heart rate during racing is substantially higher than during training at the same pace. One study, from the Sports Science Institute of South Africa, found heart rates measured in a 10K race to be 20 BPM higher than while training at the same speed; during a marathon the rates were 19 BPM higher than when training at marathon race pace. The researchers write, “The heart rate difference could not be explained by differences in running terrain or added psychological stress.” I looked up a 1995 article that found the same phenomenon; there the authors speculate that “increased sympathetic arousal in the races” accounted for the difference. This suggests that the excitement of racing accounts, at least in part, for the higher heart rates. The upshot is that using training heart rates to select race paces will lead to slower-than-anticipated race times. As a result, you could end up racing up to 20 seconds per mile slower than your pace at the same heart rate during training—and that could turn a planned 2:37 marathon into a 2:46!