Google's expected unveiling on Tuesday of a rival to the iPhone is part of its careful plan to try to do what few other technology companies have done before: retain its leadership as computing shifts from one generation to the next. The rapid emergence of the smartphone as a versatile computing device may be as much a challenge as an opportunity for Google, which built its multibillion-dollar empire largely on the sale of small text ads linked to search queries typed on PCs. As people increasingly rely on powerful mobile phones instead of PCs to access the Web, their surfing habits are bound to change. What's more, online advertising could lose its role as the Web's primary economic engine, putting Google's leadership role into question.
