I think biking helps my running. I'll walk on hills when I'm running if I start to see stars, but I refuse to get off the bike and push it. I can "redline" for much longer on the bike than running...particularly if I'm running by myself and there's no one to see me stop to walk.

I don't know if it has so much to do with the physical differences between the two than the fact that I am resigned to being a fairly slow runner but get supercompetitive on a bike (try to beat yesterday's time up that hill, or the whole route home, or whatever). Running, unless I'm in a race with people to chase down, just doesn't really motivate me.