I don't know if it's a heirarchial zbuffer or not, but new cards cheat to clear the zbuffer faster. :-) They divide the zbuffer into 8x8 regions, and set each region to "dirty" when they clear the screen, then they pay attention to which regions become cleared as stuff is drawn or something. This saves them tons of bandwidth cause they don't have to set every single z value to 0 every time you clear the screen.