We absolutely have rats that scurry along the top of the block walls that surround our yard (and all the neighboring yards). We've probably always had them, but they've definately honed in on our property since the arrival of the girls.
Although I think they are gross, and they are probably eating more food then I realize, they don't bother the hens. They only want the kitchen scraps that I've left in the coop for the hens, and probably enjoying some layer-pellets as well.
I do have an electronic rat trap that zaps a rat every couple of weeks, but we don't have an "infestation" (I don't ever see them anyway), so I'm not too worried about them. I am however, growing corn and veggies and that's when the rats come buggin! So I'll be more vigilant about setting traps to keep them off my food!!