I have yet to see Google branded cell phones (based on Android). If this were to happen, then I would seriously question what Google's objective is from a business perspective. To shift directions and get into (old) businesses that are quickly heading towards commoditization is a step backwards.

Keep in mind, their core business is advertising and as mixmax has pointed out, they want to lower the barrier to access this advertising. Trying to convince users to switch operating systems seems to me a rather convoluted path to achieving this objective.