A company like Google could probably hijack the bitcoin network. However, no matter how much hardware you throw at it, you will never make more than 300 BTC per hour. So it would be unlikely to be economically feasible to throw a giant server farm behind calculating bitcoins.
Perhaps it would become a sensible concern if a country would adopt BTC as their currency and another country would decide to attack that country by destroying their currency.
I think I might be inorrect: if somebody hijacks the bitcoin network, perhaps they could make more money by changing transactions of the past. Not really sure, though. Anyway - that would only destroy the network, not really make them richer.
