Would be nice if nepomuk could extract data from .kml, .gpx and other file types supported by Marble. See http://techbase.kde.org/Projects/Nepomuk/IndexingPlugin
To implement this, follow the steps in http://techbase.kde.org/Projects/Nepomuk/IndexingPlugin and use MarbleRunnerManager (cf http://techbase.kde.org/Projects/Marble/Runners/Parse) to open documents supported by Marble and retrieve their contents.
There's an initial patch from Illya Kovalevskyy at http://nienhueser.de/marble/tucnak-nepomukextractor.diff that can be used as a base for future work on this task.
I want to work on this task
Nepomuk became Baloo, sort of. We need some research how Baloo fits in the picture.