Best way to show map of tagged posts?

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.

Any suggestions?

I would suggest rolling your own.

Create a metabox that allows posts to store lats and longs on posts. Then create a page that finds all the posts with the tags that you want that can grab the lats and longs from each post meta data. Using that post meta data create a json variable in the html that includes all the lat long info for all the posts that have been returned. Whatever mapping library you use can access the outputted json variable as the data to use for plotting the locations.