Cliche as it might seem, the answer to my problem was to use a bigger hammer. apparently, when the bathroom was last remodeled X amount of years ago, the tile on the tub was laid with the valves in place. so the holes in the tile where cut to fit the existing valves. well, the holes for the hot and cold where fine, but for the diverter, the valve had a different shape, and the hole was not big enough to get it out. twist as i would, the valve was actualy out of the pipe in the wall, but stuck in the tile, giving me the feeling that it was still twisting on something. so, with some careful use of a hammer and a nail set, i was able to clear away enough of the tile to get the old valve out, the new valve in, and not cause any visible damage to the tile. i guess that's one more lesson learned for me, the novice of all trades.
3 cheers for a functioning shower!