if your cat eats dog food every now and then or steals a bit from the dogs plate this shouldn't be a problem though it's not ideal.
What you can't do is feed cats nothing but dog food as cat food must for example contain additional taurine as cats can't produce it in sufficient amounts themselves. In cats taurine is required for a healthy functioning of the heart, retina, bile fluid and certain aspects of reproduction.
Dogs on the other hand can produce taurine themselves out of cytein, an amino acid they take up with their food. Therefore dog food contains less taurine then cat food.
I wouldn't feed a dog cat food long term or vice versa.
Cats also require additional vitamin A in ther diet.http://www.thepetcenter.com/imtop/catsaredif.html