I was frustrated by the lack of information on the route through Anchorage. Most people must watch the start downtown, but I expect that'll probably be VERY crowded. Lacking a map of the route, I used the route information on this page to put one together on Google Maps. In some places it's pretty iffy, especially at the end, but it's probably pretty accurate in the places that spectators can reach. If you'd like to see the original, I can add you as a "collaborator" on that map. Otherwise, here are some screenshots (3 in "terrain" view, then repeated in "aerial" view).
If you want to read more about the Iditarod, the official site has info on the 1,100-mile route, as well as the history of the race (search for "serum").