For a while, Home Depot's (and Expo's) Weber Genesis gas grills came with cast iron (c.i.)cooking grates. If you look through Epinions, a common complaint is that the c.i. grates require maintenance, or else they rust.
Weber must have "got the message" because they have switched to grates that are porcelin coated cast iron (p.c.c.i.). The porcelin coating makes the grates almost maintenance free, all that is required is the usual brushing with a brass bristle brush.
So if you go shopping at HD and Expo, you will see some Genesis gas grills with c.i. grates, others with p.c.c.i. It just depends if its older or newer inventory.
Based on experience with a different brand grill, I'm hard pressed to offer benefits of c.i. grates. They do hold heat well, and cause nice grill grate marks. But the regular porcelin coated steel grates which came with my Silver-B do some nice grate marks also. So I'm having trouble trouble justifying spending $54.95 for the p.c.c.i. grates.
Also if you look at the pictures in S.R.'s "How To Grill", many of the food-on-the-grill pics are shown on a Weber Genesis gas grill with regular porcelin coated steel grates.