Google have started testing this and from what I've read not everyone is seeing it.
That's right Google's home page fades in, don't worry you are not going mad.
Basically the home page of Google is very minimal until you hoover the mouse over the page. Once the mouse is in action the home page comes to life and the fade effect will bring all the usual links into play.
There's not much I can write about this other than it's a nice touch from Google! I recorded the effect for you all to see.
Here's a quick video showing the effect in action.
Here's TechCrunche's take on it.
Update: Google have now gone live with this feature, read more about it at their blog.