The reason I didn't notice is because I haven't used the Google main page in a while. I like to have Safari's Top Sites as my home and new tab/window page, leaving me to use the search box built into the browser to initiate most searches.

When i search from the browser directly, Google's top bar is immediately visible, unlike when hitting Google.com directly. Now that I think about it, I believe I'd read something about this change (or this type of change) being tested by Google. It's not so much the speed of the fade, but rather the lack of any indication that there are more navigation options available at all until you first move the mouse. Of course with no indication that you should even move the mouse at all to possibly see them.