I ran the race yesterday and almost all of the elevation drop was in the first 1/2 mile. The last 800m was a very minor downhill, but in all honesty it was mostly flat so it really was barely noticeable. That got me thinking about how fast could someone cover 800m on the course. I highly doubt there has ever been a downhill 800m race anywhere, so I'm not sure what the unofficial world record might be. But I'm sure that SOMEONE has had to have broken 1:41 since someone mentioned earlier that the world downhill mile record is 3:24 (back-to-back 1:42s).
There are bonuses for the first runner through 400m, 800m, 1200m, and the overall race winner, so I'm sure someone might be willing to go all-out for the all-downhill first 1/2 mile if they realized they could get some prize money. I'm willing to bet that a mid- 1:40s 800m runner could run in the mid- 1:30s considering it drops about 150 ft over that first 1/2 mile. Just thinking of running 1:3x for 800m is ridiculous. Here's a link to the course elevation map on the race's website...
http://millenniummile.org/ManCourse.aspx