Besides the question of whether Google's strategy and assets have more potential than for instance Yahoo!'s (I don't think so), there are some other reasons why Google is being overvalued on the stock exchange. Nicholas Carr sums it up after an interesting remark made by Bill Gates. My conclusion would be: sell Google shares.
"The online ad market is going to become more efficient. Much of the profit that now goes to the operators of the ad-serving technology will be redistributed. Some will go to the advertisers, in the form of lower rates, and some will go to the publishers, in the form of higher commissions. And if Gates is serious - and I'm betting he is - some will go to the internet users themselves, whose clicks, after all, make the whole system work. In the battle for eyeballs, bribery can be a powerful weapon."
The more fundamental reason for the fact that the current situation is not sustainable is that it won't take long before we understand that our individual 'attention' has worth and that we are increasingly able to exploit it.