Version 3 of the Google Maps API doesn't require a key any more. Does the Google Earth API? If so, why the difference?
Update: The Earth API does require a key, though I still don't know why.
The API key is not required as of version 6 of the Earth Plugin, though the documentation was only changed to say this recently to ensure that the number of users stuck on a version pre-6 had a chance to update.
When 6.2 came out in Jan 2012, the docs were updated to show that you no longer need a key.