Google’s mobile search engine crawler now uses an updated User-Agent string designed to trigger iPhone-specific renderings of web pages. The Googlebot-Mobile crawler is building a smartphone web index based on iOS 4.1 responses including redirects. Webmasters with smartphone-specific content may finally have a reason to generate mobile sitemaps. Googlebot will skew your iOS usage numbers if not properly treated as a robot.
Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_1 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8B117 Safari/6531.22.7 (compatible; Googlebot-Mobile/2.1; +http://www.google.com/bot.html)
Google mobile search acts like an iPhone, not an Android.