From the start, Google's simplicity has been one of their more attractive attributes. Keeping in tune with that, the company has redesigned their homepage to make it more basic in appearance, hiding everything on the page excluding the logo, search box and search buttons. All of the additional buttons will re-appear if the mouse is moved over the page, quickly fading in to view.
The intention is to create a more minimalistic approach for the most commonly used Google function: search. Comments on Google's blog indicated they tried several approaches to slimming down their homepage, but the fade-in was selected among a handful of others as the least distracting.
The initial success of Google's search page was influenced by its lightweight approach. Clearly, they wish to continue that tradition with this change.