Google has announced a new plug-in and API that will allow Google Earth to be run from within a browser, once the plug-in has been downloaded. Windows-only so far (but most browsers on Windows), so I can’t add to what others have said about it: Google Earth Blog, O’Reilly Radar, Google Maps Mania, Ogle Earth.
Here’s Google’s demonstration video:
This is, essentially, of interest to developers building map applications on top of the Google Maps API, who may want this as another option. Though the fact that a plug-in must be downloaded and installed will probably limit uptake.