Well, "impossible" is a strong term. "Negative interest rates" as described are kind of a derivative of the situation, which has a lot to do with currency arbitrage, HFT shorting and other fiduciary hijinks whereby the world's wealthiest gamblers try to make money off chaos. I read a letter the other day that said anything with a positive value in Greece is a potential short right now because they may not have the ability to make a market in a year.
An increase in the popularity of these bonds indicates an increase in the suspicion that the Eurozone is going to break up. I don't see American bonds going negative because of this, but I'm an armchair guy.
I will say that if you've got a pension, negative interest rates are not your friend.