Was asked "what is the best way to display 20,000 markers on a map ?". Well, displaying all 20K map points does not make whole lot of sense, as you just see nothing, just a blanket of markers. A smarter approach is to cluster the points and display the clusters. Somebody pointed me to mapeed, and asked me to duplicate this using the Flex API for AGS. Here it is. I've loaded 20,000 map points from geonames and created dynamically clusters based on the scale of the map and its extent. BTW, this is all client side processing - gotta love the JIT in Flash Player :-)
[Update] Sooo many people asked me for the source code of the app - here it is. Any feedback is more than welcome.