The iCrossing blog captured screen shots of Google testing flipping the standard blue background line to the AdWords yellow background color. Typically, the line that contains "X results of ..." contains a blue background and typically, the AdWords ads at the top, above the organic listings has a yellow background. Here is what the Google user interface typically looks like:
But as iCrossing found, Google flipped them and it looks like this:
iCrossing must be one of the few IP ranges Google tests user interfaces on. iCrossing was also the first to notice green backgrounds on AdWords when Google was testing that.
Forum discussion at Search Engine Watch Forums.