Interesting idea.
I tried it out on my photography blog
http://www.reggie.net/photography_blog/http://ws.geonames.org/rssToGeoRSS?type=..._blog%2Ffeed%2FIt managed to get two placemarks close to Lavenham, two more rather displaced, and one as far away as Texas, USA! (NB: this info will change when I next update the blog!)
All the RSS items mention England - so it might be worth tweaking the program to ensure searches and search terms are prioritised correctly.
Here is a link to an RSS-style KML generator for a WordPress blog with Geo tags for my photography blog:
http://www.reggie.net/photography_blog/wp-kml.php