It’s a Google History: for the very FIRST time, Google search engine home page (www.google.com) is displaying 'Free Promo-Ad' (No it's not Ads by Google or an Adsense Ad) for its G1-Android mobile platform. (This may not be visible on many country specific 'Google domains' and might be visible only on Google.com domain perhaps...)
G1 is the very first Android OS powered mobile phone; it's a Google platform mobile phone. You can make search on the mobile web via Google, YouTube, Google Maps, GMail etc. and also your contacts, calendar, emails, events etc are synchronized with the Google account.
Android is facing a stiff competition with Nokia-Symbian worldwide established mobile platform and Finnish Nokia is not in a mood to 'give up' in front of Google's Android, like Internet's browser war, the O.S. for mobile phone too is hot now : )