Today, Google changed its logo to better represent its presence on platforms other than desktop PC’s — where people initially interacted with Google’s software. This is just another step in its acknowledgement that people are increasingly using mobile devices as their primary computing device. Take the following support posting that encourages people to make sure their sites are mobile friendly (and that Google will be adjusting its ranking based on this).
Google’s perspective on its logo change is an interesting read. Check it out here.