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Map plug-in Comprehensive Google Map Plugin
In struggling trying to get my site to work with the MapPress Plug-in and multiple Markers on a single map, I found the Comprehensive Google Map Plugin. http://wordpress.org/extend/plugins/comprehensive-google-map-plugin/.
Not only is it free, but it works with multiple locations on the same map, plus you have a choice of markers, a choice of using lat / long or postcode, loads of controls over the map and it even gives directions!
I've got it set-up so that I present a map within a custom display, that takes a list of postcodes from another custom display list. That way, I'm able to automatically create map markers on the fly as new locations are added using formidable forms.
I know how frustrating it was that MapPress didn't offer the multiple markers, required additional code into the header.php and also cost money for their mashup maps. This solves all those issues.
There's a lot more to the plug-in than I have taken advantage of so far, but the bottom line is... it works! Perhaps you can take a look at it and add the suggestion to existing posts looking for multi-marker map solutions