Very cool stuff!!
I have done my cornwall gallery so far...
http://maps.pbase.com/keiththomson/cornwallA few comments that I have (running firefox):
The one I would really really like, is to be able to use the middle scroll wheel on the mouse to zoom in and out.
The hover (e.g. "6 Photos") saying how many photos are at a position behaves slightly strangely. I have ended up with many of these on the screen (maybe 6 or 7) at once and it will start removing them in order as you hover over further ones. This hover also appears in front of a picture when you view the full size one.
When you click a new image to begin placing, it should be at the position where the map currently is. Too many times I moved the map away to find the right point and then found it going straight back where I was before. This shouldnt alter the behaviour for the people who like it appearing at the same point, if they dont move the map it will appear in the same spot.
It would be nice to have a mode where you can see all placemarks and move individual ones, without having to select just that one, then move it, then navigate back to the map with all the images.
But in general its excellent stuff - well done.