Until now the speed of showing markers (green pins) in Google Maps was very slow, so we were very happy to hear from Google Maps API Blog an speed improvement of 55%.
Until now we could not even think about showing simultaneously many markers in a map. You can see a clear example of the benefits from speed improvement at Revoluz (our Real State mash-up), where we could double the number of listing markers shown in the map from 75 to 150.
However there are still some Usability issues. Even if now you can display many markers fast enough, users can't select them with accuracy when there are too many together. In order to limit the number of markers you can separate them into pages. Unfortunately pagination is a metaphor that doesn't work very well with maps. It's very confusing when you see no markers where there are actually many of them. For example, the area of France can look empty, because its markers are shown in other pages, but not at the one you are watching.
Another issue are globes (aka infowindows). When globes are displayed the map moves, so you miss your location everytime you want to see a photo. That's extremely disappointing and the reason we don't have globes in Panoramio.
We have got some ideas about how to solve all these problems. You will see them working at Panoramio soon.