I fear that in practice it won't dent IE at all and only hurt Mozilla.
It will likely affect Mozilla, though for the moment it doesn't look like it, since they have extended their partnership for 3 more years. http://news.cnet.com/8301-1023_3-10028096-93.html .Though it will be interesting what will happen in that time.
Now, maybe Google Earth is not enough to make people switch, but 1 Internet user in 5 is already using Firefox with no marketing budget and no branding (vs Google being the #1brand in the world)
And if Google Earth is not enough, what about porn with no buffering? :)