Red-tailed Hawks and Common grackles are both common where I live, and it's common to see Grackles (and Red-winged blackbirds) dive bombing the red-tails in the breeding season. Here's an example of this, taken yesterday. D300, 300 f/4 +1.4TC hand-held.
Jackie, with RT's and other large raptors as long as the bird stays above the raptor they are pretty safe. This doesn't hold true with accipiters though. I once watched a pair of crows harassing a RT, we were pretty close to the nest so the RT was mad, very mad. One made the mistake of diving below the RT and the RT just turned over, a little dive and the crow was dead. The RT just opened it's talons and the crow fell to the earth.