Business Week says that mobile phone usage will be Google's undoing. This could be right - a change in how people use the web. We know Google's not threatened by Microsoft or Yahoo.
But is mobile search adoption happening fast enough? Other than the iPhone, most mobile browsing simply sucks, and people don't use their phones to buy online.
That will slowly change, and Google will have to cope with less screen space for ads and search results.
But I do think that Google has many more years of people using large screens to do most of their shopping research and transactions. And they will adapt in time to increase mobile browsing usage.