Question: I have a similar problem to [this post](https://stackoverflow.com/questions/174535/google-maps-overlays). I need to display up to 1000 polygons on an embedded Google map. The polygons are in a SQL database, and I can render each one as a single KML file on the fly using a custom HttpHandler (in ASP.NET), like this <http://alpha.foresttransparency.org/concession.1.kml> . 

Even on my (very fast) development machine, it takes a while to load up even a couple dozen shapes. So two questions, really:

1. What would be a good strategy for rendering these as markers instead of overlays once I'm beyond a certain zoom level?
2. Is there a publicly available algorithm for simplifying a polygon (reducing the number of points) so that I'm not showing more points than make sense at a certain zoom level?

Answer: 
For your first question, could you calculate the area of a particular polygon, and relate each zoom level to a particular minimum area, so as you zoom in or out polygon's disappear and markers appear depending on the zoom level.

For the second question, I'd use Mark Bessey's suggestion.
I would recommend 2 things:
- Calculate and combine polygons that are touching. This involves a LOT of processing and hard math, but I've done it so I know it's possible. 
- Create your own overlay instead of using KML in PNG format, while you combine them in the previous suggestion. You'll have to create a LOT of PNGs but it is blazing fast on the client. 

Good luck :)