This seems so simple that I'm tearing my hair out.
I want to add a page with a map showing all the posts with a particular tag. I want each marker to pop up with the title and the preview, allowing the visitor to click through and read the full post.
Ideally I want to be able to use an OpenStreetMap map as the background instead of Google.
I've had a look at geopress, geomashup, wpgeo and osm. They are all either low quality or lacking a key feature.