I train for footraces ranging from 3-milers to marathons and I commute most days. Usually I will take a day or two off from cycling before a big race. The cycling does wonders for my cardiovascular system.
+1
I'll also add that after my Sunday long run, and the accompanying rest day, my cycling improves drastically. I've seen so many gains lately in both disciplines that I'm just amazed at the benefits I'm reaping from multi-sport training. Next comes swimming...