In my opinion, this is not as bad a thing as most people forsee it to be. Take Audi buying Lamborghini for example. It is hard to argue that Audi has not done an excellent job with the ownership of Lamborghini. And most purist's reservations before Audi's buyout of Lambo have yet to become reality.
The quality of the Gallardo is world's apart from the Diablo. The service is better, and the dealership network has improved since Audi has bought Lambo.
Will these things translate to Ferrari if GM buys Fiat? Who knows. But I will say that you cannot judge GM based on their bread and butter products like Chevys and Buicks. GM is a business in business to make money. They are smart enough not to dumb the Ferrari brand down.
I think the real benefit to Fiat/Ferrari will be GM's management. I hope they will leave the engineering and styling to the Ferrari engineers, but make the company more Financially healthy. Also, things like availability and service may be improved with a GM takeover, something that Ferrari has had problems with.
I highly doubt we'll ever see a 50,000 dollar Ferrari if GM buys Fiat. Ferrari will still be a premeire mark, reserved for the elite and wealthy. Let's hope so at least.