The 51 percent threat is well-known. It was considered theoretical until last year, when GHash, at the time the largest Bitcoin mining pool, repeatedly took control of more than half of the total computational power required to create new Bitcoins. As a 51 percenter, GHash could have blocked payments from others or even reversed its own payments in order to execute a double-spend attack, which it allegedly had done in the past against a gambling site.
The group did not make use of its power, but its temporary monopoly raised alarms about the long term security of Bitcoin. Concerns seemed to simmer down after some mining pools met to discuss methods for preventing monopolies in the future.
As of now no foolproof scheme has been proposed to deal with it and, as Haubricht’s remarks show, fears of abuse are still alive and well, in regulatory circles at least.