Many elements within the
Wikipedia have geospatial locations. About 2600 places are identified in the German version, and these locations have been published in KMZ format
here.
If you read German, follow the link, click on the "Zip-Datei mit den Punktdaten" link to save the ZIP file, extract the KMZ, and load that into Google Earth. (Not yet the smoothly integrated data layer accessed by a view-based network link that it will grow into, but a nice start indeed for the very important link between the Google Earth and the informative Wikipedia project!)
P.S. The attachment is a snapshot of the data file as of 2005-07-03
Want to build a service like this for the millions of Google Earth users? Check the KML Discussion forum and code.google.com for easy to follow instructions and source code.