Until now, You could search for locations and POIs (Point of Interest) using Google Maps. What's more, if You knew the address of a hosted kml or kmz file, You could use Google Maps to show its contents. But, You could not search for contents that others may have hosted. E.g., You might be looking for trekking pathes on a specific area, or touristic map overlays (like "crosscountry ski route nassfeld austria", etc). Altough others may have hosted such info (e.g. http://adszhu.googlepages.com), You just could not find them with Google Maps.
Google has just announced the change, and, as a first step, You can query for hosted geo contents from Google Earth. Find the original article here:
Google Maps API Official Blog: Search for KML in Google Earth