I have seen a few overlays done with Google and it always seemed like the image wasn't very transparent. Perhaps the transparency can be made in an image editor and then use the resulting image for the overlay. That is a little more involved than using the tool that I use now which takes care of transparency, image scaling and rendering. I'm going to stick with what I know since all of my maps are already in that format and I don't have the time to spend on it. The aerial images will continue to change.
Dave