It isn't all downhill, but a majority of the route is a slight downhill. The route starts atop a hill that I can get up to 45mph on if I'm really cranking down it (the marathon starts about 10 miles south of me). They are most definitely not using run-of-the-mill hospital wheelchairs -- see:
http://cache.boston.com/bonzai-fba/G...04687_9197.jpg.

All that said though, the infamous Hills of Newton (a series of half-mile up, half-mile down, etc.) rollers from mile 17 to mile 21 really makes those wheelchair times amazingly fast. If you've ever heard of Heartbreak Hill (the last hill in Newton) -- that's where it is. After that, it's basically dead flat to the finish.

Those wheelchair athletes are amazing.