From the location and direction of the crack, it seems to relatively harmless in that it won't strand you with and unridable bike. However, the headset should be replaced and parts are probably not available.
Assuming the bike has a 1" threaded headset, replacements are cheap. Nashbar offers a 1" cartridge bearing threaded headset for about $15 that is excellent for the price. I have one on a beater/rain bike that has lived through three years of water and abuse and is still in great shape.