It is pretty staight forward to work with the maps API. This is all it takes to display additional information on top of Yahoo! Maps. http://api.maps.yahoo.com/Maps/V1/AnnotatedMaps?appid=YahooDemo&xmlsrc=http…. The information about what should be displayed is defined in the XML document (xmlsrc).
Here is a nice sample that maps traffic cams in the Bay Area.