This is just one method and I haven't tried it with anything more complicated than a short list of POIs.
- Go to My Maps in Google Maps and save all the placemarks on a new map.
- Click the View in Google Earth link, and then save the KML file to your computer.
- Go to GPS Visualizer and convert the KML file to a GPX file.
- Save the GPX file to your computer, and then move it onto your N800.
- In Maemo Mapper choose POI -> Import and import the GPX file.
- Done (hopefully)
